73(75)-80 core support help

[superwade57]

I know 73-74 supports are different in several ways not sure if this is one of them thus the parenthesis...that being said. I believe 3 different radiator options were available. 2 core 3 core 4 core. This is regarding the 4. Core which has a different mount system than the other two. Its a flat piece that screws/ bolts to the top of the core support into what appears to be internally threaded rivets? I'm retro-fitting a 4core radiator and mount into a 2/3 core core support. How do I duplicate/ replicate those threaded rivets? With out welding a.nut to the bottom, keeping in mind you can only access the under side of this part of the core support with the core support split into several pieces and that's too much of a PIA every time I wanna take the radiator out for some reason. Here's some pics with descriptions. Is there some sort of an expandable threadment?
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