Drum brake swap from 7 1/4" to Jeep 8 1/4"

i would suggest if you have disk brakes at the front the 9 inch drums at the rear would be good for brake bias
i.e allow those front disks to do their job before the rear locks up.

This was Aussie A body set up from 67 to 76 . for all 6s and 8s up to 360.
Later they changed the front disks to bigger with supposedly light weight and better calipers and added 10 inch drums on the back 76-82 I never found anyone who said it made much of a difference. most still home in on the mid 70s set up with the 9 inch drums and claw slider 1 pot front disc calipers as the go to set up, for retro fitting to anything else


if you have drums at the front then big ones at the back makes sense you have same all four corners and the bias is provided by cylinder bore and shoe/drum swept surface area.


if you are swapping drum brakes the whole lot needs to go across
so it comes down to whether the back plates bolt up or can be re drilled to bolt up and whether the drums fit the hub on the axle shafts, and wheel flange the PCD.

i think you will have a PCD problem did 7 1/4 stick with the 4 inch PCD
the jeep will be 4.5 with a different hub centering size if that's smaller a press fit ring can be used to center the mopar drum on the end of the jeep axle shaft

presume 7 1/4 has a smaller axle housing end than an 8 1/4 so

1) luck: and the 4 bolts that hold on the back plate just match axle to axle and Mopar used the same dimensions
2) luck: the distance between back plate and axle wheel-mounting face is the same axle to axle i.e mopar persisted with a dimension from housing end to axle shaft mounting face between the 1970s and the 1990s otherwise the drum skirt will rub the back plate or the reverse happens and you have a great big gap.
AND
3) no luck: and they don't match but from smaller axle to big may well give you the option to redrill the new pattern into the back plate provided it doesn't have some wierd contour just where you need a bolt. You can of course off set the back plate to 1 o'clock or 11 o'clock to facilitate re drill if need be, just need slight adjustment of brake pipes to reach.
4) No luck you do all of this and then discover that the drums sit too far in or too far out and rub the back plate as mentioned before... or they don't centre

custom axles could sort that.....

Dave