Hi blacklemming
from the engine description you mention (Buick 215CI) i believe you are talking about the veritable Rover 3.5 V8 ?
If so, then the cheapest way to increase would be to fit a 4.6 Rover V8 or even a 5.0 litre version - all aluminium and you wouldn't in theory need to change the manifolds, or gearbox etc.
worth asking the folks on the UK forum (
UK Cobra Club - The Front Page) regarding the larger Rover V8's or even speak to DAX themselve.
rgds
Gary