Hey Mike. Can't help you any on this one. For the BritishV8 market the 350 Buick isn't likely to gain much if any support. Based on my experience with the 340 it is just big enough to cause problems that the 300 does not have and the 300 can be stroked to 350 cid easily. So there is no advantage, and there are no headers for the 350, where everything needed for the 215 through 340 and all Rovers is widely available. The difference between the work needed to fit a 350 and a 455 is almost nothing as well, but just about anyone considering either of those swaps is more likely to choose the Chevy LS instead for fairly obvious reasons since the work involved is the same.
Sorry to be the bearer of bad news. I know you've sold a few sets of Rover heads to the BritishV8 market and from all appearances they should continue to do well there, especially on the larger displacement builds. But I wouldn't count on any 350 sales there.