The 3.5 and the 3.9 have the same size exterior engine block, onlt difference is the stroked crank in side.
You can get carbs on any rover v8 engine so it may be standard but it may not. Eather way they are piss easy to strip and rebuild (to standard) and arn't that expensive eather.
What you would have to find out is what the compression ratio is or failing that, what the first 8 engine numbers are and i can tell you what the engine came from as i have a chart with them all on but its a bit long to write on here.
They put them engines in everything, sherpa vans, TVR's, boats, everything, even NASA had a rocket with one in i think (don't quote me).
They ranged from 3.5 to 5.2 i think and depending on the compression, can go like stink or a bag of s**t.