Welcome.
As most everyone else has stated, if you don't have the time / heart to tear into the Monte, park it on the side and get another DD or sell it to another relative who TRULY understands that you've got first dibs on it when if they want to get rid of it. Oh, and make sure that relative will actually take care of it and keep it like you guys have. I like the sleeper idea myself, keep the color, interior, even the stance and build up a nice powertrain for it.
Depending on how you build the 350 it can be efficient too as long as you don't go crazy with it. Put on a properly functioning Q-Jet and with those small primaries you can still get good mileage. Add on an overdrive transmission and you're looking at even better mileage.
In terms of the 267, it's an OK version of the SBC but that's about it. Pops had one in his '80 wagon and considering what / how much we hauled, # of road trips and the how it was mistreated as it got older (lack of maintenance) that little engine put up with a lot of abuse for well over 150K miles. It lasted longer than the 400 in the '74
Impala he had!! Anyway, just run it as is, continue to maintain it until it gives up the ghost.
Put it this way, whether you stay with the Monte or go with the C5, you're going start spending money because it's really hard to keep them stock forever. Do you have a winter beater because a Corvette in the snow :shock: (from experience)