I'm in a similar position myself. Unfortunately any of the few decent V8 cars I've seen the owners won't part with 'em!
I've done a bit of homework & spoken to a couple of people who've worked on them so have some answers. I believe that any v8 from an SD1 is the motor to go for - although I'm not sure if the sump had anything to do with it.
The bulkhead does need chopping about a bit - but not a lot. Also for access it's tidier to chop out the slam panel between the headlight housings.
Fortunately the Chevette transmission tunnel is very large so apart from welding brackets you should be okay. The LT77 'box is from the SD1 but I don't see any reason why a R380 from a Discovery can't be used - this is a popular conversion for MGBV8s.
I assume you know you'll have to change the back axle (a hard driven 1.8 ohc Cavalier engine will destroy it).
I don't know if you want a roadburner or not but here's a few things to bear in mind before you start spending money (& before I start slagging the motor off I should say that if anyone's got a decent converted car I'd be interested in it for sensible money):
The Rover V8 is a very old design engine (Noah used it on his ark) with poor power figures - a normal V8 SD1 only had 150bhp and when shoved into an MGBGT only managed about 135bhp due to poor manifolding! This is probably what you'd get in a Chevette - a Vauxhall Red Top will beat that. Even the Vitesses only managed 190bhp.
There's any number of better engines & it's a very, very expensive engine to tune for decent bhp (assuming that's what you want), see here
http://www.jeengineering.co.uk/ Of course, if you really want a V8 why not think about a Ford 'small block' 302 - about 5.0ltrs but it's both narrower & shorter.
Anyone have a 6R4 for sale? Unfortunately I've only got £1K spare but I could throw in a few eggs to sweeten the deal!