Are you trying to put a 1-piece seal flexplate on a 2-piece seal engine or visa-versa? That won't work. The balancers will interchange and almost everything else external, but the flexplates have a different bolt pattern. DO NOT make the holes bigger. If you have a like for like swap it will fit. Even 400 and 454 will fit-they just won't be balanced.
As cheap as a flexplate is, just go buy one for your new engine. What year is the 305?
Your 82 350 is a 2-piece seal, the end of the crankshaft has a flange on it like this.
http://www.thirdgen.org/forums/tech-...-will-fit.html
A 1-piece seal crankshaft is round all the way. like this
http://oppositelock.kinja.com/1pc-or...siv-1705492654