Help radiator is a too short.

[Pontiac1976]

Are you working with a 81-87 rad support, LMC part # rad support upper bracket (side mount) LH 32-0942 & RH 32-0943 use with lower rad that do not sit flush rad support light duty Rads.

LMC part # rad support upper bracket (top mount) LH 32-0944 & RH 32-0945 used with tall RADS like the 73-80 heavy duty 305-350-454 two three core

All tall one will be around
19"H x 28-1/4" W x 1-1/4" D
20-5/8"H x 28-1/4" W x 2-1/4" D

All short one will be around
17"H x 28-1/4" W x 1-1/4" D
17"H x 28-1/4" W x 2-1/4" D 2 or 3 core

old style copper Rads if you look in the cap area cooling tubes( core) if they are open then it's a true cooling tube if it cap tube it's not a true cooling tube.
so it no fluid can go throw it, and it still has three tube. It really only two core Rad IE: two tube but no fluid can go throw it, and it still has two tube. It really only a single core.