If the 74 has a 4spd without overdrive, it is pretty easy to
find an M41 in a salvage yard. The M40 -> M41 swap is dead
easy relative to most other transmission swaps people talk about
here. And, M41s are easy to come by; be sure to grab the
driveshaft while you're getting the transmission.

I'd say, though, the first order of business should be to
drive the car for a while to find out what else is wrong with
it. A 74 shouldn't be as loud at highway speeds as my 70 and
68 164 and 144s were.

A 74 should be a really nice car. I'd fix what's wrong
with it, then try to find a 75 ignition system, *then* look for
an M41 to give it a nicer ride. Oh, and IPD sway bars and
bilstein shocks and probably new ball joints / tie-rods.
chris