I am converting from Bravos to Transmissions with surface drives and need to know should I have extra mounts put in to support the transmission?

I am guessing arnesons? I would think you will want rear engine plates mounted between the bellhousing and block to support the rear half.

Exactly aas Thunder put it. You should have a rear motor plate between the block and bell housing to support the rear of the engine (Obviously, you need a front motor plate as well). If you do this, you do not need any additional support on the tranny.
I have seen Arneson setups that don't use a rear motor plate but use the mounting pads on the tranny to secure the trans and rear of engine. This was an ASD 6/7 with the tranny part way through the transom and into the extension box. It had mounting ears (aluminum angle) attached to the transom and to the tranny.
Eddie

If you are using the Bravo/Arneson Conversion kit, it comes with rear engine mounts.

what transmissions ? if using B/W they have mounts right off them...

That depends. The TRS tailstock had mounts built into it that bolted to the inner transom. The speedmaster tailstocks did not. The speedmasters either used ears that bolted to the sides of the transmission bolting to the inner transom plate, or a rear engine plate bolted to the stringers. I do have two sets of those ears for the speedmaster style mount I want to sell. I dont think they will help this guy out with the arneson though. He is better off with a rear plate i think.

I was already going to use rear engine mounting plates. It would seem a 150lb transmission hanging off the bell housing would be to much.

No , with rear engine mounts ( RACE MOUNTS ) u be fine !

This is how its done in all the transmission boats accept the older TRS set up. ( like Thunder said )
They had 2 ears coming off that where bolted to the inner gimbal plate.
Which was/is a PITA !

No problem as long as the engine is supported front and back. That rear mount is taking a bunch of the tranny's weight.
Eddie

Yes, definitely that's OK. All Merc driveline setups as well as inboard BBC setups using BW 72C's, even with the extra reduction gear cantilever the transmission off the bellhousing. Structurally, the bellhousing could withstand thousands of pounds with no problem. Inboard boats end up taking up most of the propeller thrust into the bellhousing as well.