I presume you have a GMC truck for the GMC engine.

I got a new front pulley/harmonic damper for my 302, same as 270, from a 292 Chevy engine, same shaft, keyway dims; 2 groove pulley, 3/8 belt. I have heard of 3 groove pulleys, but didn't need that. Machined a little off the end so it would line up w/ 302 water pump, alternator. I couldn't find another modern harmonic damper.

'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.