Jack, I went thru this last spring, mine's an '84 as well....
as to the thickness there, don't know...three methods from my study/time on the forum:
1. Brass tube method from CD and others, requires a flanging tool, and allows you to get very close to the cockpit floor which is ideal.
2. Gene Neill's method, drill your holes close to the floor as possible, bed the transom core with epoxy, finish off inside and out, best method in my opinion, but quite labor intensive, messy, etc.
3. Marelon thru hull fittings, easiest, less messy, but not large openings as others, what I, clouddiver and others have done, sealed in 4200, you can't get 'em entirely on the floor due to the flange, but sufficient backup to the factory mess IMO, about an hour's work.
I'd guess the transom is about 1 1/2-2" thick, fiberglass/plywood/airspace/plywood/fiberglass.
Suggest you do a search for transom scuppers, Gene's are the cat's pajamas IMO, he'll post his sweet method w/pics, my lazy man's method is below: