Just doing exactly this job. I lifted the body off the chassis but because of head room in the garage I couldn't get it high enough to slide the old out.
Instead I chose a line missing all of the fittings and bolt holes and cut in two to then slide out with the body back in situ.
template trial fitting perfect fit Instead of maring ply I used Birch ply as it has many more layers of ply than marine and as a result is a lot heavier as long as you waterproof the edges with a 2k wood filler.
varnish undersides ready for install I have then applied 6 coats of varnish to the underside along with all the usual POR15 paint on the underside of the lips where the floor bolts to.
I am hoping it will last the 60+ years that my original did ;-)
