What's the axle center register size?

[375InStroke]

What's the size of the center register on large bolt pattern axles?  I assume it's larger than the small bolt pattern.  Do other axles have different register sizes?  Let's say one gets a set of 11" backing plates off a 9-1/4" diff, would the drum be too big?  I got a set cheap, but the register looked huge, so I thought I would try the backing plates with a set of '60s C-body drums.  Maybe it was my imagination.  I'm used to looking at the small bolt rears.

[PureGTS]

  The register is 2.825". Small bolt circle is 2.320"  The backing plate problem is a little different. There are three different distances between the axle housing flange and the back side of the axle itself. To check this measurement remove the wheel and drum and stick a tape measure through the largest hole in the axle flange until it hits the backing plate. Then measure the donor the same way. If they match you can transplant the brakes between them. If they don't the shoe may be too deep or too shallow either way causing problems. Drum hits sides of shoes or jams, or the drum doesn't hit all of the shoe.
  If my memory serves, the A body 8 3/4" was like 2" (the smallest), std B body  2 1/4", and the third I don't recall. It may have been for the early B body 8 3/4. Bare in mind this information is from the days when all of these vehicles were available in the junkyard for this type of inspection at the same time. What this means is there are at least three different brake shoe sets possible. The truck 1/2 ton, 9 1/4, 11x 2 1/2" were the same as the B body's if I remember right too. But measure it just to be safe. And watch the bolt circle for changes too.