How much power do engine mods yield?

GEARZZZZZ
Bestest bang for the buck!
3.55s over 2.76s is a 28% improvement in torque.
To get that in the engine will cost many times more than gears.
It does make the hiway ride a little buzzy tho.
City driving will be a blast!

Stock /6 cams are lo-perf. Very lo-perf. But if you upgrade them, without in increase in compression, the Dcr goes away, making take-off with the same gears, even more sluggish.So then the gears are mandatory.
Might as well start with gearz and a sharp tune-up. Maybe that will be all you want. If its a city-only car, you could consider even more gear. But if you are sticking to low diameter tires, dont go too far, as first gear rapidly becomes useless. I don't know whats available for 7.25ers. I think 3.55s will put your small-tired Valiant at a tic over 3000rpm at 60mph,depending on TC slip.
I would forget the M/T swap, as you will lose the off-the-line snap, that a T/C offers. A slightly looser T/C might be nice, but do the gears first and see if thats enough of an improvement for you.A looser T/C will mostly be felt just in the first car-length,or so.

Do not be misled into chasing horsepower. Horsepower requires rpm. Rpm has all kinds of requirements.Mostly expensive, and all destroy low-rpm torque, requiring even more bandaids on a DD or cruiser. Torque is what you are after for that slanty. And thats exactly what the factory built. You just need to bias the output to the chassis to satisfy your personal performance need. Gears will do that. Effectively,Quickly and Cheapy. Good luck what ever you decide.