If those tubes are less than 12" and the motor is only 1/3 hp it's ability to dry will be questionable even in a 1300 bu bin unless you get lots of really warm dry weather. It might be time to look at a more dependable unit. Obviously there's lots of full floor units out there with big fans but I'm thinking that might be more than you want to spend on a small bin. Half round above floor vents are an option, but they are a pain to clean out around. I've used All-size above floor vents for years with good results but I don't think they make them any more. Similar idea to the half rounds, but more triangular shaped. The only problem I have with those is that the original bin entrance was only a 6" round pipe, so now I have to make my own entrances for the 9 x 14 fans. If we combine in Aug, I start binning at 18% moisture, and I've used 3 and 5 hp fans for up to 21 ft bins and 4400 bu. With canola I start at 10% and aereate until I cool it regardless of moisture. Often it takes longer to cool it than dry it. Years ago there was a guy with a story in farm show magazine that used a flat entrance on one wall... like a register to get the incoming air into the bin. With a good moisture proof vent to exhaust the wet air, I bet that would work fine for your 1300 bu bins. I just use a couple sections of strait above floor to get the air in. Once it's in, the grain determines the air flow, not the vents. For the 19 and 21ft bins I use more vents and configure them in an X or a Y to get more even air flow, but the 14ft ones are pretty easy to aereate. There's a new concept being floated around now that says you should blow the air into wall vents, and exhaust out a center vent (maybe a use for your existing system?) which makes a lot of sense. The big limitation to aereation is outside humidity and condensation on the bin walls when temperatures start to drop. The condensation would be less of an issue with the wall entry concept.