A relative cheapo like a Triumph or "maybe" an AH, perhaps.
Still, bet anyone that did that is still kicking themselves for ruining the investment they had.
I’m not sure there are many XKEs with American V8 conversions. Mostly it was done to later Jag XJs because their resale value was so low (due to poor reliability).
Drop in a small block Chevy, like a 283, and it would loaf along at 90...
When you get west of the Hudson, in Indian country, you used to see hundreds of miles long stretches of open road... TR-3s, TR-4s and 4As with blown engines, (Engine bearings were notoriously thin.), were cheap, light, had good suspensions, and had the room for a small block.
You could get sets of oversized bearings from Warshawsky's, out of Chicago. After turning the crank shaft mains and the con rod bearings down for the third time, a V8 started looking good...