The SU's (especially the later HIF ones) aren't too bad for fuel, as far as a carb can go I suppose. Leyland claimed 80 MPG for their Metro HLE (1.0 A-Series 998 with carb) in the 80s. The MG Metro (single carb, same power as a Mini Cooper S) managed 40+mpg with care, tricky in a Cooper S.
Still the Metro (as in Suzuki) motor may be a better option if you have access to more of them
Its just you have to change a load of stuff for FE ? High pressure piping, swirl pot etc. I tap from no experience of having done (or not done) this though.