i have looked at a few pictures of peoples longtails and there cav plate looks like it has a small L shaped piece bolted to the end of the plate what is this special piece for is it needed

It depends on the maker, I figure you are going to make your own? The L shape you are refering to pushes, in effect the prop and cavation plate into the water. Counteracting the lift effect.

but by looking at the cavitation plate it is already angled up in the back so it would appear that already would be forced to be pushed down by the water passing over it as the water passes over it does the extra lip just add more of that push effect ?

correct !

so with knowing that what is the benifit of the extra downward push if the existing cav plate is already pushing down ?

On a GDLT, there is an extention of the cav. plate that you bend.

On a MBLT, mine atleast, you dont bend the cav. plate there is the peice bolted on to bend. It allows for better fine tuning on the thin metal and if it gets bent and damaged it is easier to replace.

ya mine just comes off i dont think theres any way to bend any of it by the way its made i guess im gunna have to make one like yours