Assuming you have MGB 4.5x14 wires fitted (as I do), rear wheelarch clearance depends on four factors: tyre size; number of leaves left in rear spring; rear lowering block choice; and the weight of the occupants.
With 175/70/14 tyres, the top three loose leaves removed from the rear spring stack and a 75mm lowering block I had no clearance issues whatsoever. Ground clearance and ride quality were a different matter, however - I had to crawl over speed bumps and at 50mph on 'B' roads the car was so jumpy and skittish it was too scary to drive any faster.
To address these issues I fitted: softer, 150lb front springs (cut down by a coil and a half); 175/80/14 tyres; a 50mm lowering block over the diff; and removed another leaf from the remaining spring stack.
This made a world of difference to the ride and handling and I had no clearance issues with just me aboard, or while driving my (then) 12y/o son about. Then I took an adult passenger out for a drive on the bumpy B roads round here and found that the nearside rear tyre fouled the arch over every bump. Thankfully it didn't do any damage, but it sounded and smelled awful.
Taking the lowering block out raised the rear by a couple of inches and sorted the fouling issue out, and, with a few other tweaks, my car can now be driven with real vigour on any road surface and at ridiculous speed with complete confidence.
So, the suspension setup I now have is as follows:
Standard dampers (new);
150lb Spitfire front springs cut down by 1.5 coils (170lb would also be fine);
7/8" Spitfire anti roll bar;
Spitfire steering rack (quicker than Herald/Vitesse but not too heavy with a 14" tiller);
Blue poly bushes all round;
Three loose leaves removed from the rear spring stack, plus a further leaf taken out of the remaining stack;
No lowering block, diff clamped directly to spring;
4.5x14 wires wearing 175/80/14 tyres (20psi front, 22 psi rear);
Toe in on rear wheels, toe out on front wheels.
On that last point, I haven't actually measured the angles, I just trial and errored it until it stayed straight as an arrow at high speed but turned in with a lovely balanced feel when you began to give it some steering input.
Others may prefer a stiffer setup for the track, of course, but for the road I reckon I've got mine just where I like it now.
Good luck getting yours set up how you want it.