IS a floor pan cut to length to suit a swb J&S the same length as a Manx floor pan?

YES

I would do a quick measure first
some buggies were 12 1/2 n some were 13 1/2 inches, in the early days .... so get yr tape measure out

Manx floorpans are shortened 14.5 inches.......
80 inch wheel base

early days ...... what were they poppy ???

14.5 inches ... now there is an interesting paradime....

there you go 2 inch differences already ...
I think the J & S were 12 1/2 from memory

It wont hurt to run the tape over the body first

So how do you determin this?.
Do you measure from the centre of the front hub to the centre of the rear hub?.

i'd measure pan length from where front goes up over tunnel to where back goes up over tunnell.

mi wheelbase is metric 2075mm but thats useless info as its a hand made frenkenstien.