Q1 What is a good choice for a reliable 15" mid motor?
My choice would be a rebuilt triple OMC big foot similar to the code red motor

I don't need max HP, 50+ would be fine. Can older motors be re-painted to look good? A pretty boat needs a nice looking motor
Use a later model Johnson or Evinrude cowl, pan and skirt.

Q2 What is the small offset jackplate of choice? I am not sure that I would put a jackplate on it.

. I think I know that the 49 cube OMC triples came with the 15” nitro gearcase, but without power trim,
The 1973 -4 49 did have a short shaft model with the big foot. It was a push pull shift. The nitro came out in 1975. If you wanted to spend a lot more money buy a 15 inch with a nitro gearcase. If you wanted the best motor buy an original 1988-9 Johnson Formula E.
The 15 inch didn’t come with power trim, that’s why people use a Yamaha Pro 50 clamp bracket and power trim assembly. You just switch the steering arm. There are videos on the site about the code red project and Pro 50 trim. Look under Gordon Racing.

If you wanted to get a 50 hp 15 inch with factory there is a Yamaha listing on this site. There is also a 40 mercury 15 inch with factory trim.

Quote
and the 56 cube motors had the 20” 2.4:2 ratio case.
Yes, same ratio is on the big case short shaft.

What is the big foot?
It’s the larger gearcase, same as the long shaft model.

Did it come in 15”? The boat I am pondering is 15”