Cool, thanks TH! He lists these (3+1/4" as well as the 4+1/2" 100-6 and 3000 size). The problem is that I don't have any of the originals to compare against. The core itself is 4" thick, the front cover is about 1/8" thick and I have no way to measure the rear plate but guess it's another 1/8" to 1/4" thick. Therefore I think I would need a clip that is around 4" in the relaxed position, which falls right in between the two sizes he offers.
Just a little background here, I've owned this car for about 18 years and it never had a heater, just the three-pronged mounting plate bolted to the inside if the fire-wall but no core, motor or cover plate. This car used to be my Dad's. Anyway, a little over a month ago he calls me and says "when I was getting down the Christmas decorations from the attic I found a box of Healey parts, should I just throw them out or do you want them?". Uh yeah dad, just pitch 'em ;-) So anyway, in the box was the heater core and cover plate in decent condition but no motor, fan or clips. I decided I really didn't need the heater functional but would like to restore the cover plate, paint the core, get some clips and put it in there as a non-functional cosmetic piece.