It's your build, if you've done your homework and a 73 to 77 Monte Carlo frame will fit and that's what you want, then do it, it's yours. As far as having a modern suspension with the Monte frame, well I guess that's another point of opinion, they make many improvements now for the Monte suspension.
If you buy a donar Monte Carlo, it might have a 350 engine and tans in it already and that would be a plus, they can be picked up relatively cheap too. Some Monte Carlos had a pretty plush interior for it's day as well, I had a 76 with white swivel buckets, console, tilt steering and a full gauge package. The more I write the more I like the Monte Carlo swap, just with the suspension upgrades.