Hi Glenn, welcome to the forum. Lots of P5 owners and lots of Rover techy help here. And I see a few freinds already!
I'd go for a 4 ltr, Glenn. then you can use the far superior Coscast block as a start point. No problem using the SU's and manifold with that capacity, although the SD1 manifold is marginally superior to the P5/P6.
Far more important than new engine is to do something about the gearbox though! The ZF 4HP22 is a straight slot in and you can use the original change lever. Then you have an overdrive top and decent change quality. Not to mention much better economy thanks to lock up in 3rd and top. And it is a Rover item (sort of) as this box was used on the later Range Rover Classics as soon as it became available.
Now how about some "before" pictures, so that we can see where you are heading back to!
Chris