You will not have balance problem. Ive added split collars before to help rigid the cylinder. DO NOT WELD TO THE SHAFT, ive seen that done before by accident. Static balancing of the cylinder is dont further out towards the bars. What little mass you add with welds are not going to make any difference. Etienne you need to attach a full set of rasp bars before welding in the collar to locate the bulkhead/spider. Preferably new bars that have not been warped,bowed, stressed.
buddy i had a customer have this same problem, he had it welded, the next harvest when he went out and ran a rock threw it, it screwed everything up. i had to replace, new spider, all new raspbars, and filler plates, new concave and a new beater grate, if you gone taking it all off that far you might as well get a new spider, but the worst thing is if you do that you have to take th shaft out on the side that you broke that spider. same spider broke for him. and that shaft sucks takes out, a lot of panther piss
I service a lot of combines a year and see 2 or 3 same Rh spiders broken------have repaired by welding fabricated split hub which is clamped to shaft like driver spiders are and then welded to spider--we have these hubs in stock at store----or can weld ring to spider but ring will be loose on shaft and should be temporary---but have had no vibration issues either way whats so ever and never have removed shaft and replaced spider for this broken ring
I've welded a lot of spiders or cylinder heads, sometimes with the feederhouse on, never had a problem, you need to put the rasp bars on and make sure that they are were it was from new, you can see the old weld and weld it at that spot. Also if you want to make sure it's balanced there is a way to do that.