Given the walterline length and displacment of the boat you can
calculate the horsepower required to go at any speed. The waterline
length and displacement can be found by taking measurements off the
hull, or they might be avaiable for the Thunderbird somewhere. I've
only done it for sail power, not outboard motor power. I don't know
the difference between rated engine horsepower and thrust or whatever
number measures the actual power applied to the hull. The point is you
can calculate to find the best outboard for a boat.

That's the speed in still water in a dead clam. Wind, tides, waves,
and current will effect the actual speed. You might need more speed to
overcome local tides but a more powerful engine won't make much
difference because it's the waterline length and displacement which
are the biggest influences on boat speed.

Of two engines using the same engine block, the one which produces the
required horsepower at the lowest rpm's will last longer and burn less
fuel. On second hand engines the other main factor is the cylinder
compression which tells how worn the engine is. Measureing the
compression is cheap and easy to do, but find out if it has a bypass
to make pulling the starter rope easier. Lawn mower engines have this
which makes it impossible to measure the cyliner compression.

There is a well-known exponential relationship between power and speed
on a dispalcement hull. The point of maximum curvature where the power
requirement takes off is the "hull speed", equal in knots to 1.34
times the square root fo the walterline length. Commercial operators
run their boats at about 60% of hull speed for best fuel economy.