Todd, Is your car posted on the BritishV8 site? I'd like to look at it.
The Rover motors go as high as 5 liters or 300 cu.in. as used in some TVR's. The deck height of the Buick 300 is about 5/8" taller and it needs a different bellhousing. The deck height of the 340 and the 350 is 1-3/16" taller. This means that these engines are a little wider. The 350 would need different exhaust manifolds. The 340, probably the 350 and possibly the 300 uses different engine mounts and the attachment points to the frame may be different. The engine compartment in the Special is smaller than the larger Buicks but it's a good bit bigger than an MGB and we put a 455 in one of those. I don't think it would be very hard to swap in a 300, and if you paint the block silver it takes a very knowledgeable person to spot the difference. The 300 can be built to 350 cu.in.
Oops, you said Skylark. Should be able to fit a 455 in there. Probably can get the frame attachments off another car.
JB