The difference in just the powerheads will be in the 50-60 pound range (obviously the 70 is heavier). I think you will find without doing some major re-fitting and jury rigging that you will never get a 70 powerhead to mate up with housing from a 50 - they weren't from the same production family (40, 45, 50, 60's shared parts) and (70, 75, 80 and 90's shared parts), but they didn't cross back and forth.
The 70 would need "it's" exhaust system to produce the 70 horses - even if you did get the 50's housing retrofitted so that it would bolt up, it would restrict the exhaust enough that you would never make 70 horses (so you would also have to refit the exhaust plate) - and then using that to run a jet unit would drop your horsepower a further 1/3.
A 70 horse jet, running properly, roughly equates to about 50 horses at the "prop". So anytime that you don't NEED to run a jet (due to very shallow water or for safety reasons), you should stick with a regular "prop" lower unit.
A "JET" sounds fast, and there is a "cool factor", but horse for horse they produce about 1/3 less power (at the prop end) than a standard outboard...