I am building a LT for a tall transom with a 31hp vanguard and wondering how long the shaft should be. I know MB and GD make LTs for tall transoms, anyone know how long the shafts are and what angle they run the joint

Some basic slope formula will give you an answer.

A 16" transom is a 72" shaft.

so you get 4.5 inches of shaft length for every inch of transom.

In theory a 20" transom will be 90" with the same angle, most I have seen are 84-88

well thats easy enough, thanks

How big of an issue is running a regular shaft long tail on a tall transom?

It will be a PITA to keep the prop in the water without modifying the cav. plate.
The angle the prop goes in will be steeper so it will make the bow run low. (plow)
You loose some of the reach the motor would have.
The tiller will be higher than a comfortable level.

I have a 20hp GDLT with a 72" shaft, on a short transom the handle is very low and you have to take a knee to run down the lake. I had this motor and a tall transom boat and the handle was the right height. Serial number is 10622. I got a boat with a 16" transom with the understanding that I would get the better performance out of the motor. I am 6'2" and it seems the handle is too low now. Is it possible I have a tall transom model or handle? Or is this just how it is supposed to be?

you're 6' 2", the average height is 5' 8".

the handle isn't too low, you're too tall.

the handle can be bent or welded to your height. you will get better performance on the 16 inch transom, though it won't be a whole lot.

DA BOAT'S TOO SHORT !!!!