270's or 302's or 235's all sound about the same, so a noisy(rod knock, etc.) will sound the same in all. Lots of reasonably priced GMC parts available. 270's and 302's aren't especially rare, just need to look in right places-farm trucks, milk trucks, ambulances, etc.

Any automotive machine shop can do the crank, cylinders, etc.

Last edited by EdPruss; 11/26/2015 11:22 PM.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.