as you say, it's never going to be original. and, Buick Specials really don't have a lot of collector value anyways. so the answer is, do whatever the heck you feel like, it's not really going to affect resale value that much.
so far as the Rover engines go, you can get a newer engine ( up to 2004 ) that displaces 4.6L, uses a serpentine belt accessory setup, 6 bolt / cross bolted main caps, gerotor oil pump and EFI+coil packs on each plug ... and it's pretty much the same size as the 1963 Buick 215ci / 3.5L, although the coil packs are going to stick out a little bit.
otoh, the Rover guys LOVE the 300ci for the displacement bump and the fact that the iron blocks/heads don't have near the problem with head seal and thread stripping that the alum parts do. the crazier guys have been known to drop a Buick 350 crank into a 300 block and get a short deck 340ci. the only problem is getting a 4 barrel intake to feed a 300 deck height performance build, that's a 1965 year only part unless you build a sheet metal intake. it is possible to use a Rover intake on a 300 but you have put spacer plates under it.
be aware that your transmission will NOT bolt up between these engines. check out the bellhousing thread, the Buick 300 uses the BOPC bell housing while the 215 / Rover use a one off pattern that GM didn't use for anything else. so if you're going to swap engines you may as well make it a transmission swap at the same time.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Vladimir Lenin
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)
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