Neale, I also restoring a Franklin Arrow and replaced the drop axle.
I think that it is a good idea.
Because i have 3 brand new tyres I left the 13" rims and brakes but if I have to do it again I will upgrade to 14" rims and tyres on Ford bearings.
This will improve the weight cargo capacity of the van.
Regarding the plywood floor before you go north get some oil based decking oil and give the ply on the need few coats.
My son, also restoring his old Millard and in his YouTube page you will see several videos of the work that have been done in both caravans.
The videos are here:
Rover Tasmania