For info, approx V8 bhp figures
3.5 - 160-190bhp (dependent on version, most were the lower power ones, higher power were the Vitesse engines)
3.9 - 188bhp (Range rover engine, basicaly 3950ish cc)
4.0 - 188bhp ish (Range rover engine, basicaly 3950ish cc but, cranka dn oil pump etc changed, slight block mods like cross bolts on mains)
4.2 - Rocking horse muck!
4.4 - 200bhp
4.6 - 222bhp (Stroked version of the 4.0)
5.0 - 340bhp (TVR engine)
All are fairly rough figures but, show its not an engine that produces much bhp per cc, it is good for torque though and hence why it eats the pre95 Range Rover gearboxes if taken off road....
And I havn't mentioned any of the specials like the twin plenum 3.5s either as you aint liekly to find one!
Note also with a Rover V8 engine, you are very likely to need to re-build it as there not hugely good at high mileage with cam, follower, timing chain, sprocket and valve gear wear being all to common.