A recent MGA! tech session addressed just the issue you ask concerning
putting an MGB front suspension under an MGA. NAMGAR has a website and I
believe you can search for technical articles and order copies at a very
reasonable price.
The gist of the article was that there were three major issues involved.
First, the shock absorber and top trunnion mount is different between the
two. You can either open up the mounting holes on an MGB shock to fit on
the MGA (there may be some sheet metal issues as well) or you can use
spacers to adapt the top trunnion to the MGA shock (the MGB trunnion is
considerably narrower). Although the article did not mention it, there may
be camber issues with this solution.
The second issue is the position of the steering arm. Apparently you have
to shorten the MGA Rack by some amount to allow proper toe adjustment. I
don't remember how much. As an aside, don't be tempted to mount 14" MGB
rims on an MGA. The front rim flange can rub on the tie rod end. When you
are buying an A, look for this as it would be an expensive surprise.
The third issue is the brakes. Apparently the MGA uses differant fittings
for the hose and the solid line when compared to the MGB. Some custom
fabrication would be required.
Concerning setting up a Ring and Pinion (crown wheel) from scratch. Back
in my mispent youth, I was an avid reader of Hot Rod Magazine. They had a
technical article on installing used ring and pinion sets. To arrive at
the proper setting, they suggested that you smear the teeth of the ring
(crown) gear with grease and rotate the pinion. You should be able to tell
where the ring and pinion are meshing by where the grease is rubbed off the
ring teeth. If the center of the rubbed spot is to the outside of the ring
gear teeth, the pinion is shallow and has to be spaced further back. New
gear sets should have beautiful oval contact area right at the middle of
the ring gear. Older gear sets may have a different contact shape but it
should still be centered. Once the contact area is centered, adjust
backlash as required by shimming the ring gear carrier.
On a personal note, I had a very old Jeep truck whose rear end howled like
a banshee. Since the truck was pretty much worthles, I just loosened the
pinion nut a little and let the gears find their own working arrangement.
It worked for the next few thousand miles before the truck went on an
extended holiday due to something else breaking.
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