outboard bracket

Hello fellow boat builders.
Im going to attempt to build an outboard bracket for a 150 to 200 hp engine for my 20' b/w. Ive looked at several of the brackets that are out there, and cant seem to put out over $1,000 in one.
What I have now is an I/O and want to convert to an outboard, one reason is the I/O is seized, second, i want to put a sofa bed against the transom, and free up walking room as the engine cover is rather large and I only had 2 lounge seats.
As for the bracket i was looking to make it out of diamond plate, fiberglass, and angle iron, I also want it to have a swim platform, but what im asking is, how will diamond plate last in a salt water environment. Will it rust. If it will , what could i use to attach to the angle iron as a plate and platform that will take the pressure of the high output engine. Any help would be appreciated.
Please email me also with suggestions at acdntpron6@aol.com
http://www.kerbeck.com/transombracket.jpg

Dig in your pocket and buy a pre-made aluminum outboard bracket like the B-Bracket. You'lll be glad you did.