Welcome, BlueRunner :)
Yes, I think twin 140’s are too much for that boat.
Assuming the hull is structurally sound (?), I don’t think the coast guard rated it for that much horsepower, then or now. Maybe they allow for more hp for twins than a single motor - best check that out. Could be an insurance/liability issue.
Also, the weight of twin 140’s may put your scuppers underwater when the boat is at rest. I don’t know if twin 115s will save you much weight.
I love the idea of twins on a V20, especially if you are running offshore. Are you?
Since you already have the boat, the first thing you need to do is see if the boat will plane off with just one engine running at a time. Twins become a disadvantage if you cannot plane on one. The surviving engine will burn too much fuel on a long run back to port. You would be better off with one big motor and a 9.9 hp kicker to bring you home just in case.
My V21 takes a 25 inch shaft. The cavitation plate on the motor should be about an inch below the running bottom in front of the lower unit. Twins can go a little deeper because the boat will rock side to side in substantial seas. You must keep the water intake submerged, but too deep will cause excess drag.
Good luck and enjoy your V20!