18 ft transom replacing

[sponge]

Hey guys, im replacing my transom on my 18.
measuring the wood it looks like 1 3/8 does that seem right.
so i guess i need a sheet of 3/4 and 5/8.
its going to really tight to install.Maybe I should go thiner.
Is it ok to put transom in in pieces to get it behind the 3" flange.
Doing transom from outside.