Somewhere on one of these forums, I saw a picture where someone had lain a piece of plywood over the seat riser extended out to the firewall. You'll need to put a couple of supports for it towards the front equal in height to the seat riser.
It formed a nice flat surface from the back of the cab to the front on which to lay while working up under the dash. Having rewired my '49 a long time ago, I remember thinking it was a truly outstanding idea that would make the job a BUNCH easier.
