Octane was way below 87 or 85 when our trucks were new. Whatever the lowest grade at the pump is adequate, anything. Higher is just a waste of money. No alcohol is best, 100 low lead adds more lead than we need, probably not legal if anyone is looking. 80 octane avgas, if one can find it, is OK.
NO alcohol is best.
'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.