Current front suspension is set up for WIRE wheels.
Your problem might be your solution.
You're running GT, so you can run knockoffs. Have your wheels built around the wire centers. I've got spares, if you need them, and if you're REALLY SERIOUS about this, I'll send you a pair for the cost of shipping. It's pretty straight forward to an experienced wheelsmith, and you'll be able to adjust your backspacing. The Australian Healey team was running knockoffs at WOS last year.
OR
Wire wheels have been used in racing for years. Go with some Healey BN7 wheels. They're 15", 4.5" wide, and they fit on the MGB hubs. I've got them on my '72. Get some tubes and a set of frontrunners, and you're golden.