305 or 350

[Shanker1371]

Ok Recently I bought a new flywheel for my what I though was a 1980 350. The truck the engine came out of was an 86 GMC longhorn with a th350. I am using a 3spd. I went through some trouble because the bolt pattern was off. The last three of the block number is UFC which comes back to an 80 350. However the the flywheel that was on it was externally balanced which wasnt how it was in 1980. So I traded for an 86 flywheel and it matched. I have been told that all 1/2 ton gmc came 305 in 86. The motor has been freshly rebuilt because when I pulled it I found it has almost new everything. This is also what the owner told me. Remember I'm a wrecker driver and I was told this before he knew I was buying it and I fell at the time he had no reason to lie to me. Somebody please help me.