I remember a friend put a Rover V8 into a Bedford CF a few years ago. Also I've seen a Jag V12 complete with rear end in a Transit (the engine was next to the driver though).
Assuming you have space in the engine bay, a Ford V6 plus gearbox can be picked up for £300-400 from a specialist scrappy. Then it's a case of fabrication of replacement engine / gearbox mounts, modifying the propshaft and reconnecting or rerouting things like exhausts (single or twin pipes), radiator hoses, cabling and electrics. A reasonable Ford 3000 V6 should give you maybe 150-180 bhp. Rover 3500 V8s start at 180 bhp and up to 300 bhp or more for a 4.6 ltr.
Basically, in cost terms it amounts to the time spent carrying out the swap plus the buggeration factor of sorting out the above items.
Myself and my comrades in arms are kit car enthusiasts (cobras in particular) so we all have spent time scratching our heads over strange car building problems, so nothing is insummountable.
As they say "There's no replacement for displacment". I have 6556 cc of displacement

(and the fuel bill)
