N6 cage diameter

They are the same diameter. I have a P3 rotor with channel helicals in my N6 and would never go back to the P1 and flat helicals.

You will have to make little blocks with holes in them to mount the channel helicals as the cages have different mounting holes.

The blocks go on the outside of the cage to hold the bolts where there is just a square opening. I have the 68° pitch in my N6 so the holes built in for the flat helical bars do not line up.

You want the thinnest layer of material going through the cage. You also want a little disruption in the separator end to shake loose any entrained grain. The cross bars in the separator grate do that in the newer machines. With the older machines without a grate a couple of F2 rasp bars positioned at about 8 and 4 of the belly of the separator end help to stir the material.
Also, a "hyperized" P1 with P3 stars in the separator end to give a high/low tumbling effect or a P3 rotor with every other row removed in the separator end.
Reverse bars impede the flow through the cage and cause the material to bunch up and the rotor tends to grind that plug which then causes more MOG on the shoe making it more difficult to get a clean sample.

Also with a thin layer going through the cage the grain has a better chance of falling through the cage because there is less material for it to "hide". This is why a gentle "flipping" and "stirring" is more efficient than reverse bars and provides a cleaner sample.

The P1 with P3 stars in the separator end giving a high/low tumbling effect works very well with the F2 bars acting as a stirring device. The reason for the channel helical bars in the thresher is to get the material away faster to reduce over threshing and keep the MOG inside the cage reducing the load on the shoe. With channel, you have twice the gripping area to keep the MOG moving. Plus, with the steeper pitch, the MOG moves father each rotation.
The problem I found with the P1 was the rub bars that were not aggressive enough and would loose the grip on the crop. The rasp bars on the P3 rotor are more aggressive so they hold onto the crop better thus reducing the residence time in the cage and reduces the potential of the MOG getting ground up into small pieces causing more shoe load and eventually a dirty sample.
The key here is to thresh it and forget it!

FYI - More residence time inside the cage does not guarantee getting all the grain out of the MOG. Good aggressive grain threshing grain at the concave, then quickly moving the MOG to the separator to be gently "stired, not shaken" to remove any entrained grain without breaking up the MOG is the key.

Couple things to make a P3 work.
1) you need a 3/4" spacer behind the coupling to get the rotor centered. If you do not have that spacer in there the bars will rub on the gearbox wall. Also by extending the coupling you will have more output shaft splines inside the coupling to spread out the load on the coupling splines.
2) you will have to put the stop sign bearing on the inside. The P3 rotor shaft is shorter and will almost be flush with the the lock collar.
3) you will need to shim the speed sensor wheel and modify the pickup sensor.
Make sure you have a "Hyperized" P3 rotor with every other row removed in the separator end and in the blanks area a discharge paddle backing bracket in the mounting holes to help stir. I hard faced mine for longer life.
Also, you need extended cylinder bars into the discharge area on at least every other row. Make sure you have channel helical bars going into the discharge area. (see the pics on the hypermod site)

There are obvious little things that will be need addressed such as remount the coupling grease fitting and turning the bearing around in the pillow block bearing housing on the discharge end.

I have been told that wide spaced bars in a P1 do not work. I think it is because the rub bars are already not very aggressive. The P3 bars are similar to the conventional rasp bar design and a wide space bar works very well. This is what I have in my P3.

Just an FYI - I had very good results with a "Hyerized" P1 rotor in corn. I was not happy with the P1 in green stem beans even with channel helical bars throughout the cage. I determined that the P1 rub bars were loosing the grip on the bean straw. That is when I "Hyperized" a P3 rotor and put it in my N6. I got an even better sample in corn and little to no howling in green stem beans. I believe it is because of the more aggressive design of the rasp bars.
If it were me, based on my experience, I would replace a P1 with a "Hyperized" P3 and channel helical bars from the get-go.
JMHO
Pete Hinrichsen

I will have to defer to NDDan (Dan Hurrt) as I do not have wheat so my opinion would be just that..... an opinion.
I would think that filler plates would be your best bet as I believe that the purpose of the filler plates are to give more grain threshing grain area to help get the wheat out of the head. You should not loose anything your other crops with filler plates. I am afraid that you will if you put the narrow spaced bars in.
The salvage N6 that I am scraping has a concave with filler plates in and it did only corn and soybeans.
Just my opinion and not based on experience with your crop
Pete

This reminds me of the old Combine Talk Show days. Wow! What a site with legends like NDDan, Tbran, Hyper Harvest II and others. These men took a cob grinding, power robbing almost worthless pos and turned it into a incredible, highly efficient machine that would compete with the high price spread.
These men spent countless hours refining what they believed to be a good concept but hindered by things such as reverse bars which on paper made sense but in application failed miserably. It was engineering that built this machine but it was also that same engineering that did not understand what they had built.
It took the Combine Talk Show gurus and others to release that monster residing within and then share it with us at no charge!!!
Thanks guys.

I am going to look at the salvage N6 that I have and it has a hinged door that flips up when you need to get the rotor out instead of taking the whole sheet off. Dave Moeller did this to a lot of combines. I will post pics tomorrow. Not sure if you would be interested in it or if it would be cost effective to ship it to you. Check out the pics and see what you think.