Hi.
I managed to get a set of cloth seats from a classic RR. They have a bar running along the back that was too long for a P38 but if you cut it down to size it fits behind the rear seat catches, you can still lock the rear seat and then fasten the frame of the 'dicky' seats to the floor. Lap belts (recommended for rear facing seats) can be added to the anchor points under the rear seats.
I hope this makes sense. So if you want a 7 seat P38 don't discount the extra seats from a classic.
Keep on bodging.