Having rebuilt a '63 e-type drophead down to the last nut and bolt, I can say that the Jag straight six is a lovely engine. It's no lightweight with the cast block, and quite tall as pointed out above. Somehow the company squeezed them in there back in the day though, and she was a very potent car with the side draft webers - somewhere around 350hp I think. I can't recall if it was a 3.4 or 3.8 or what in there, but a 3.8/4.2 with webers would have plenty of poke and sound amazing. Durability was a D-type strong suit as well, hence the very solid performance at the longer races.
Can't wait to see Fran's gang getting a few of these on the road (and track) - what a treat.