Re: My first boat - 1982 Starcraft SS160 rebuild
My $0.015:
It might be great ply to build other structure out of, depending on how you plan to seal those, but for the transom, I'd probably go 1 sheet @$49..... It will do the job & it will be mostly unseen...
The BB will likely have a much smoother face on both sides then the fir does on either 1, if it's ACX. Which for console sides, seat bases, platforms, etc may make the BB easier to finish, and finish well, possibly w/ 1 less coat of epoxy &/or spar and ultimately make a smoother final finish for vinyl, but definitely paint.
The other point would be is great (better?) ply worth $30 more then good ply to you? The fir ACX prepped, sealed & cared for will likely outlast the 30yr original. So the extra $30 might be better spent elsewhere....