One peice main...
86 and up one peice rear main seal motors flexplates have a different crank bolt pattern--wont bolt up to an older two peice rear main 85 and older crank..
I had this dilema when I put the 86 305 from a G10 van in my 79 C10 ..the 79's 250 six had the "3510" style starter,the same starter most 305's and 350's also used--I figured it would bolt right up! WRONG!..it would not bolt up right to the 305,its nose hit a "bulge" in the oilpan rail that the older small blocks never had,due to the new "one peice" seal design..the bolt pattern was the same "staggered" pattern,but the 305 van starter was notched out on the nose to miss that oil pan bulge,and I noticed the nose was tilted where the drive gear came out,not straight like the 3510 was..
That might have been irrelevant though..
My flexplate was the same (larger)diameter,being a van one,not a car..I can see why your having trouble,if its a car flexplate,you'll need the matching starter to go with it..in days of old,there were only 2 starters for a chevy--one with staggered bolts,for the 168 tooth flexplate,and the "straight across" bolt pattern for the 153 tooth flexplates..now there are several different choices,and many look nearly identical,but wont interchange..
DON'T run only 1 bolt in a GM starter,EVER!..it will snap off in the block,and possibly crack the block too!..get both bolts in there before its too late!..It could strip the flexplate and crack the starter nose too..anything is better than NO bolt in there..

..having that "brace" on the front of the starter to the block is a good idea too..
