Opportunity for a full powertrain transplant

[Zogbert]

I've got a 1972 C-10 (250 CID with three on the tree), that I'd like to use on the highway. It has a few issues. On any acceleration, and above 50 mph, the valves rattle like crazy. I've checked the timing (many times), burn premium fuel, and have experienced no real change. Also, the gearing is far too low for what I want.

I've got an opportunity to purchase the engine, transmission and rear end from a 1976 Chevelle Malibu with 55,000 miles.

A) Will this combination work in my truck?
B) Am I going to solve any issues?
C) Will I create more issues?
D) Is this complete sacrilege?

Thanks for any input!
Zogbert