Hey JClark, You're better off putting a 4.4 litre Leyland P76/Leyland Terrier Truck alloy V8 engine in. Those Leyland V8s were exactly the same as the Buick Rover alloy V8, but they had a block that was slightly larger (which meant if one was to use a Buick or Rover inlet manifold in it, one would need spacers on each side of it, between the manifold & the 2 Heads). As such it has the potential to be bored/stroked up to about 5.6 litres, while the Buick/Rover versions really only have the potential to go not much further than about 4.7 litres.
Also the 4.4 litre Leyland varient had the rockers mounted on stud bolts rather than on shafts, so if one wants to do a roller rocker conversion, one could use a slightly modified Roller rocker kit for GM small blocks (Holden or Chev). Plus the bottom end was much tougher as they were designed for Leyland Terrier Trucks, plus as they were designed & built in Australia for Hot Aussie conditions, the water passages are wider so they stay cool better. They also come with a twin plan manifold designed for normal downdraught carbies (such as Stombergs & Carters), so its a simple matter to modify it for a 2 Barrel Holley 350/500. Either by using an adaptor plate or through a bit of grounding out & alloy welding, but its best to keep the dual inlets under the carby seperated, so one doesn't loose the twin plane effect (where each cylinder in the firing order takes its fuel from the opposing carby barrel), which aids low down torque tremendously, without effecting revs too much. My P76 manifold was adapted to take a Holley 650 4 barrel (vac secondaries, the extra 2 barrels only go into action when rev it) without much hassle. If one was to do such a conversion its best to dyno-tune it & re-jet the carbie to match the engine, to get the best efficiency & power. The Terrier varient has a low reving torquie cam, so it really should be re-ground.