Thanks for the sequence advice and reminders Berlinista.
I am confident the 3M + 1/4in foil faced foam will stay under the 1/2"+ that the original bitumen & jute was. Here's a pic from when it came out of the car (good riddance).
I think I'm going to get the bitumen down - partially to protect the floors so I don't scratch them during reassembly - poke thru all the bolt holts, then wait on the foil faced foam. I'll likely cover the bit with cardboard and do the headliner etc.
'73 GTV bare metal project
'87 Carrera 3.2 Daily - '65 912 project in waiting