Mine is a 6A and there are now three 6A's in Newcastle with V8's, we did have a 5A but the car died - its V8 is now in one of the other 6's. 
My engine is out of a '67 P6 so its a bit tired.  All I have done to it since it was fitted over ten years ago is fit a new cam and new lifters. 
The other two cars have more tuned engines, one has the 3.9 engine with 270 BHP at the rear wheels and has been clocked at 153 mph
( at Brands ).

The only mod you need to do to the engine is to fit a P6 water pump to line up the pulley’s.
My car does not have power steering but Chris Burrow can advise you on that fitting as his has.

All three have the SD1 box and SD1 prop but two of the bolt holes at the diff end have to be redrilled as the Rover fitting has a square fitting between the holes ie the holes are equal distance apart.
I rotated the old gearbox mounting plate 180deg around its rear bolt fixings and drilled and tapped new fixings to the rear. The SD1 5 speed box with its mounts now drops onto the plate but needs a spacer to lift it slightly ( about 1" ) ( understand that ? ) 

The engine mounts need to be rotated until they are square with the chassis to take the Rover engine.  If you can get P6 engine brackets the engine mounts stay where they are length wise along the chassis - if SD1 engine brackets they are further back on the engine.  You will need a remote oil filter, mine is fitted between the engine and the nearside chassis, on the 5A he fitted it in front of the radiator on the offside.

All of this is fairly straight forward, your problems start with the manifolds - you will need to get some made or have the P6 Rover cast manifolds modified ( the SD1’s would be harder to fit ). The mod is simple in that the flanges are cut off and refitted at an angle which will allow the rest of the exhaust system to be mated but being cast you will lose some power against fitting stainless tubes.  The nearside is not a big problem but you have to avoid the steering rack on the offside.

My manifolds are not very good quality and I intend replacing them when I find someone to do it at a price I can afford.  You will also have to lose some of your footwell space but only on the drivers side. I cut away the footwell vertically at the “seam” next to the clutch master cylinder and “squared” off the area behind the engine.   Last big problem is the gear change. 

The SD1 change extends too far back for the Scimitars.  I managed to find a change mechanism from a XJ6 which uses the same box with the gearlever bolted straight onto the box, with this the gear stick is in the same place as the original.
XJ6 manual changes are not easy to find, I believe the Sherpa van also used the same box with a similar change to the Jag's but not so refined.