The biggest problems associated with propane powered buses were the lack of power and the lack of range. We did quite a few propane conversions back in the '80's but were rather disappointed in the results. Part of the problem was IHC stopped making gas engines in the early '80's. If your fleet was nearly all IHC and with the diesel option having a payback around 50,000 miles there really was no reason to go over to Ford or GM chassis in order to get a gas engine that you could convert to running on propane.
Fast forward to now and all of the bus OEM's offer a fully supported factory propane option. The option has eliminated some of the problems we had back in the day with conversions. But they still have some issues in regards to range. On a route bus range isn't so much of a problem because you can fuel every day or twice every day if you have to do so to get there and back. But on a trip bus it can still be a challenge to have enough range for a round trip or finding a fueling station near your destination that will fuel a bus. Propane fueling stations are becoming much more common but they aren't always at every freeway off ramp or available for 24/7 fueling.
The Ford propane buses have a problem in that there is no Allison transmission that will turn as fast as a V-10 has to turn in order to get any power out of them--the redline is well in excess of 6,000 RPM. As a consequence the transmission Blue Bird is using in their propane powered buses is a Ford transmission. While the 6R140 is an okay transmission in a light duty bus or truck, it is really only an E4OD/4R100 on steroids. It isn't close to the Allison transmission that Thomas and IC put in their Type 'C' and 'D' propane powered buses or the Allison GM puts in their G-series van-cutaway chassis. They also have had some real teething problems and very few Ford dealers are qualified to work on the Rousch conversion. It may say it is a fully supported factory option but finding a dealer who can work on it can be a problem.
The Thomas buses with the 8.1L propane power use what is basically the GM big block truck engine that has been converted to run on propane. It requires a special Allison transmission which will tend to make it more $$$$ if anything should go wrong with it. Because it is a GM big block it will turn considerably faster than most other medium duty engines which is why it requires a special transmission. But since the redline is basically the same as what it was when Allison made the AT500/MT600 series transmissions that went behind the older model GM big block 366/427 engines it is doable for Allison.
The IC buses that are propane powered use a purpose built 8.8L propane/multi-fuel engine built by Power Solutions International. Externally it looks as if it is a GM big block but it is a ground up development by PSI. It looks like a GM big block because some of the design engineers at PSI cut their teeth working for GM. The design parameters were the engine had to have a power and torque curve that was similar to a Cummins ISB. It was also designed so that it can use a standard off the shelf Allison transmission. When it was installed in the IC bus part of that design parameter is the bus would have to have a minimum round trip capability no less than the same bus with an ISB diesel engine. It also could not take any more time to fuel than an ISB diesel powered bus. Both PSI and IC have been able to meet those design parameters and IC has been building lots of propane powered buses since they started making them available for school year 2015-16. https://www.icbus.com/bus/articles/power-propane
Power Solutions International, Inc. – Cleantech Engine Manufacturing
Propane powered buses might actually end up being more common in the future than diesel powered buses. Unlike diesel powered buses propane powered buses will start up in subfreezing temperatures without requiring any block heaters or other starting aids. The fuel won't turn to jelly when it gets cold. The engine will put out more than enough heat you won't need to have an Espar or Webasto fuel fired auxiliary heater. Propane fuel is so clean engine oil change intervals are stretching out way past an annual oil change--it is possible that in a 13-year life cycle the oil may only need to be changed five times. Part of clean running hype is propane powered vehicles do not have to use DEF or EGR systems in order for the tail pipe to pass sniff tests. The exhaust does not stink and it is very clean. And for a lot of schools a big selling point is propane is an American fuel that we don't have to purchase from countries overseas.
Would I use a GM 8.1L propane conversion in a bus? It would all depend upon what I planned to do with the bus. Propane shines the brightest on short trips or trips that can be accomplished in a round trip without having to stop for fuel.
If your plans for your conversion is to never go more than a couple of hours away from home a propane powered bus might be the answer for you. But once go outside of the round trip circle and you have to start searching out in a strange town a propane fueling station that will fuel a bus a lot of the reasons for having propane power will start to wane.