Hey, Kev -
Having done one on a '69, I can tell you that it's not that bad. The rubber bumper Bs are easier, as the firewall has more room, motor mounts are different, and the radiator location is more forward.
On the chrome bumper Bs, you have to cut away the brackets where the motor mounts bolt up, relocate the radiator, and "dress" a couple areas of the firewall. Also, the steering shaft u-joint is too large and you'll need a smaller one, as the exhaust header will foul it.
Earlier chrome bumper Bs (pre-68, I think) also have narrower transmission tunnels, so that's an issue.
If you haven't already, check out British V8's website for lots of examples -
http://www.britishv8.org/Photos-MG-Conversions.htm
And info:
http://forum.britishv8.org/
Oh, and the GM 3.4 found in early model Camaros and Firebirds is the engine of choice, along with the Tremec T5 transmission.
Bill Guzman, at Classic Conversions, is the go-to guy for the installation kits, as well as lots of info:
http://www.classicconversionseng.com/ Brian, at BMC also:
http://bmcautos.com/
Good luck and let me know if I can help.
Mickey