For the rear, I don't want to do an axle swap.
If you don't want to do an axle swap, you're stuck with the 10" drums. The 11" brakes cannot be easily swapped from a JB6 axle (e.g., from a junkyard) onto a JB5 axle, as the bolt patterns (and perhaps a few other things) at the end of the axle are different JB5 vs. JB6. The easier route is to swap an entire rear end.
There are disc brake "upgrades" for the JB5 axles. Some are junk, and some are better, e.g., see post https://www.gmt400.com/threads/little-shop-mfg-rear-disc-installed.48588/post-1252997 and others on the forum (@Schurkey often posts to the forum regarding brakes, he's a good guy to follow). If you get a disc brake conversion kit, get one that's got newer-style calipers mounted on integral brackets (as I noted in my post above) and not a cheapie kit that uses a the older-style (70s era) GM calipers. You'll often see me and @Schurkey in the forums speaking-out against those cheapie kits.
(edit) This kit, for example, used the newer style calipers and has a parking brake provision. I'm not endorsing it, I'm jus' sayin'...
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