My dad had a(old) P reg, in 76/77 is that a mk 1 ?
If so it had the extended nose to start with until a woman backed her allegro into the front so was repaired by her sons firm, that did camper vans and came back with a flat front.
It had a 1700cc V4 engine though, that's all I can remember as I was 8/9 at time, loved it though nice van.
Good luck with the engine swap.
tabetha