Ben, a long action M995 in magnum caliber will stop into a TRG42 chassis perfectly. You need to use a TRG trigger pack, since the m995 trigger falls short in TRG42 chassis.
To share my experience on a similar project a few years ago: I took an action from a M995 (375h&h) and had Tim “Moon” Robert to install a 7RM barrel. I managed to get TRG42 chassis from a guy who ruined the plastic furnuture on his TRG42 (300wm) by cerakoting and baking them. He sold the aluminum chassis to me. I was able to buy a TRG trigger on gunbroker and a new set of plastic furniture here. The final result was a TRG/TRG-S hybrid in 7RM. The cost of the project exceeded the cost of buying a used TRG42, and it took me long time to sell that rifle at loss after I decide to switch to AI platform. But it was fun.