In my not so humble opinion (IMNSHO), I would definitely use 6" x 6" lumber
to achieve the working clearance you desire (may not give you 24" but I'd be really uncomfortable raising it much higher and being underneath it) but from my experience, will give you plenty of room to work and inspect. (Trust me, I've spent hours under them.).
As for rebuilding brakes on a '69 with original axles (again, speaking from experience), I cannot imagine a bigger waste of time or money. I did it once on a '72 Overlander and really wished I'd just put new axles on it (or left it alone). With the '73 Sovereign
, I replaced the entire running gear, which gave me new suspension, brakes, and NeverLube hubs, not to mention brake drums with studs rather than threaded bolt holes (for my new aluminum rims).
Even with removing wheels while elevated, you can accomplish the "lift" with the 6" x 6" ramps on one axle, while using jackstands to stabilize the rest of the coach.
New axles aren't that big of a deal. I replaced mine with Dexter 3600 lb axles
for less than $1000. Modifications were minimal. But it addresses a host of issues and should give another 15-20 years of service.
(3 years ago, I would have fought that advice, but after piecemeal approaches which added up $$ over time, I've finally seen the light!)
PM me if you have any questions.