65 gmc build help

[andyjust4today]

Hello, I'm new to this site and am looking for some general advice. I have followed some past threads on doing a 1965 gmc 305v6 conversion to a sb 350. I have started my build with the short block off of a 93 tbi 350 and bored it 30 over with double roller timing, stage 3 cam and lifters, hv oil pump, holley dominator intake, and edelbrock carb with the bear can ports... the heads came off of a 81 I believe. Ok now for the dirty. Everything went together fine engine is pristine and all but once I got into the build Ive started to watch as the budget disappeared. The perches and mounts I bought private party I was told would fit the 65. Not the case they were off of a 73 with the clam shell mounts. First off engine sat way too high in the front and the hei would have never fit next to the firewall. Also the block huggers wouldn't fit because th clam shells where in the way, go figure right. after some research I went and purchased a set of 65 c20 v8 perches and mounts and with a little maneuvering and some new holes drilled in the frame engine sits perfect and block huggers fit. Now for the dilemma, I have a sm 420 granny low that came with the truck and due to the outstanding hauling performance I would like to keep it. The bell housing I purchased for this build was off of a 65 L6 which I have been told should work. Not only is the transmission now a pain to get lined up (had it in and out of the 305v6 in 30 minutes before), now when I get it slotted and start to tighten the 4 bolts it doesn't want to meet properly. I figured we maybe its just a tight fit on the bell housing's center hole so slowly but surely got it to mate..... Bam flywheel will no longer turn with the stater on otherwise. Pulled the transmission back out and the flywheel turns. ok... I purchase a the correct clutch set for 65 c20 v8 along with pinion bearing etc. because at the time I was using the clutch set from the 305 v6. With the new clutch installed go to put sm420 back in and am having the same problem. What went wrong?