a 250 would use the same tranny as a 350 V-8 one nice thing bout chevys that both inline 6s and SB V-8 have the same bellhousing except for the pre-'60s? of course i bought a '84 250 for 75 bucks for my '64 ran fine when i bought it but i let it sit a little to long and then it didn't keep an idle just kept rackin up the rpms but ya now the motor ain't being used so it was a waste, if you want it to be factory in 64 you had the choice of a 230 I-6, 292 I-6, or a 283 V-8
Nate
