Welcome to the site Al. You're running into issues because the combo you're attempting to install never came in a 72 K20 from the factory. A 72 K20 with auto trans would have come with a TH350/NP205 combo. Your TH400/NP203 combo is a squarebody era drivetrain. So you may indeed need a custom crossmember to mount your combo - and I don't know the effect of the TH400/203 combo on mount location in a 72 etc. Was the truck apart when you bought it? I realize I haven't answered your question, but at least now you know why it doesn't fit
. While the TH400 is a popular upgrade to a pre-squarebody 4x4, the challenge normally lies in mating it to the NP205 - an adapter is needed. TH400/203 combo retrofit is not common because the 203 is notably weaker than an original 205 (chain vs gear driven). 203 into a pre-square is usually only done out of necessity - i.e. it came with the truck or someone had it lying around the garage when their 205 grenaded (which is hard to do...) Hopefully someone here has swapped a whole squarebody driveline into this era and can provide some more useful info for you. In the meantime, it might not be a bad idea to keep your eyes peeled for an NP205 t-case that someone is wanting to sell.