Can I really pull the heads without pulling the blower? If so that is just great!
The heads can be removed without pulling the blower. Clean the area between the heads and blower well before disassembly.
Whether or not to do an inframe depends on what the bearings look like after you have the one piston/cylinder assembly and rod pulled. Did the bus hold good oil pressure? The bearings are not expensive, but you'll have to be certain that you correctly fit new bearings if you change them. The "O" rings are another issue. I'd be tempted to replace all cyl "O" rings. If one failed, the others may be right behind them. As has already been covered, if you don't intend to do rings, don't pull the pistons out of the liners.
What to replace and what to leave is the sort of "judgement call" that brings Mr. Murphy out for a visit.
Do you know the history of the engine? The "O" rings may have been overheated and damaged (or improperly installed...or just old and tired)...while the bearings and rings may be fine. If it wasn't smoking when hot, had 15 or 20 lbs of idling oil pressure when hot, and cranked good cold...an in-frame may be overkill.
Do you have any rear engine (flywheel "rear") oil leaks? A high milage DD will leak oil at the rear main. If the rear main is leaking, you may find it much easier to pull the cradle, and work on the engine out of the coach. The rear main could then be replaced. The cradle with engine/trans and all the filters and coolers is easy to remove.
You cannot remove the oil pan with the engine on a dolly, but a dolly sure makes removing it easy. The oil pan is easy to remove in the bus, but working on the heads is a PITA. You'll be standing (squatting) on the muffler heat shield on the drivers side...that's gonna be fun. The pax side head will be easy enough to remove in the coach. Every time you bump the upper engine compartment, dirt will fall into the engine. Steam cleaning the whole area may help.
You could set the cradle on cribbing, or build a dolly that would fit the cradle rails which would allow access to the lower engine. Should you decide to pull the engine and use the cradle as a stand, be sure that the cradle is well supported under the area where the motormounts are welded to the cradle.
If you pull the engine, that would be a good time to replace all the rivets that are probably missing in the engine overhead panels.
There is a bit of skill involved in doing the work you are diving into. You may be a mechanic sort..but if not, get some experienced assistance. DD 2 strokes, contrary to popular opinion, are not simple things to repair. You'll see what I mean when you read the service manual.
Sorry about the long windedness....nothing going on you know.
Good luck, JR