Im not running exhaust through drives ever, took ram horns off, got blockoff plate from Merc to seal from the inside.

I was thinking about putting a rubber cap on the oval through hole in the transom assembly, and plugging the 1" side ports with freeze plugs. That assembly is painted but cast aluminum right? Should I use stainless steel plugs, or no because of dissimilar metals, find aluminum ones, or just use brass?

why? they dont do anything if you put the block off plate inside and they are not in the water when running. blocking them off will have zero effect on the way it runs and I have never seen the block off plate leak.

That isnt in the water at all? I just figured redundancy is good, if something were to leak. never going to need the exhaust through drive again, and if Im 30 miles out I dont mind two things keeping water out

Totally agree and a waste of time. Nothing will be accomplished period.

What I do for blocking the exhaust off, is make sure the transom side and the plate itself is flat. Always use the Merc sealing o ring as well and make sure its a new one. Even when taking off and putting back on --- A new sealing oring. BUY the Merc one

The merc plate and o ring is whhat i used, scotchbrited the surface before putting on. Didnt use any sealer should I have or just o ring?

Get a wooden dowel about a quarter or 8th inch smaller than the hole. Wrap duct tape till a tight fit. Silicone and bang em into the ports. Seal off with silicon or epoxy. You change your mind they'll come out...

Its a simple 1" hole, just need to know what best material freeze plug would be best. getting them out wouldnt be hard from the inside taking the plate off if you really wanted to..

I wouldn't use a freeze plug. Diff metal corrosion. Non-metal plug is best way to go.

Ayuh,...... The block-off plate, inside is all that's needed,.....

If ya also block off the outer ports, you create a cavity in the transom, which will get water in it eventually,......
The followin' winter, the water will freeze, bustin' yer transom assembly, causin' all sortsa problems,.....