yu need to go retrieve your tape measure
measure radiators overall height of tanks at the seams, and also measure width form seam to seam centers, these measurements will tell you what will fit what, ignore anyone who says they are the same even two and three core ones, i have run across about anything you can run across as far as variety of heights and widths of radiators from 67-87, its more interesting that you would think, for sure,
there were several widths and heights depending on drivetrain ordered and HVAC system ordered and all kinds of other factors,
shrouds changed in 81 anyways regardless of any thing at all.
4 core is unique shroud and mounts, can be a PIA to convert without a 4 core parts rig especialyl 81-up
if you need a factory 73-80 four core rad top panel then lemme know i have some of them still, and i have 67-72 ones too, no 81-up though, when i put a 4 core in an 81up i just modify the bvrackets with tin snips and a hammer and a carpenters knife, works fine
if you want details then lemme know and ill explain
the last 82 305 th350 truck i put a four core into i used a 74 C20 454 th400 one and it worked perfectly.
'67-72 used smaller radiators, those trucks didnt use as big of radiators
alot of the 77-79 trucks with 350 and smaller engines used radiator that was waaaaaaaaay low, the bracket panel was mounted down on front side of the radiator support well below the top surface of the rad support, even with a 350 yes and even in 3/4 4x4's
i dont know why GM did this, maybe they ran out of larger radiators, 77-79 was mass production group years it seems, but they also used shorter 52" rear springs too under 3/4 tons those years alot,
good luck