I would not even consider putting that kind of money in a 40+ year old boat. If the floor has given way, it's likely there are other structural failures also. 50 bucks spent on an inspection by a competent mechanic may be money well spent.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner