MG, eh? That marque has a grand history at Bonneville going back to the late 1930's.
B? Midget? BGT? A? Which one you starting with?
Sounds like you're heading into Modified Sport territory, although the MGB-GT might fall into a coupe class these days - 1 5/8 cage would keep you legal at those kind of speeds - and the Nissan is a killer motor.
Texas would be a fun event for such a car. If you're thinking Bonneville or Maxton, get a rule book ASAP, and I'm really not kidding. You can wind up a long way down the wrong road before you realize you need a map.
I encourage you to check out Hotshue's build diary (Bonneville Bugeye), and if you'd like to know how NOT to do it, you can check out mine (Milwaukee Midget).
Boy Howdy - welcome aboard, mate!
PS - Need some parts?