RE: 1990 school bus
FIRST: You have changed this vehicle to a much higher RPM engine that it was originally. You must perform a driveline analysis to make sure the driveline configuration you have will not be torsionally active at this higher RPM. Dana has a good driveline calculator online called Dana Expert. If you do not do this, you risk driveline vibration, or worse consequences. If you don't know how to do this, or don't own an inclinometer, get some help from a qualified driveline tech.
Here is what is in my opinion, the safest and most cost effective method for this conversion after you make sure your driveline can handle the increased maximum speed it will be turning:
You need to look at the data tag on the transmission and record:
1. Trans Model number. Should be AT-545 or MT-643
2. Serial Number
3. Assembly (or part) number
Go to a transmission rebuilder, or an Allison outlet that sells ReTran transmissions. Use your transmission for a core and buy a transmission that is built to go behind a gas engine that matches the full load RPM of your engine. That way you make sure you have the right torque converter to match the engine and the correct shift points.
There are other ways to attempt this. They involve choosing the right torque converter, valve body calibration, output governor and governor drive gear on the output shaft of the transmission. It is time consuming, risky if you miss something, and may end up costing nearly the same as buying a remanufactured transmission.
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If you just experiment with different springs for the shift signal valves, you may or may not end up with satisfactory results, and the question of proper torque converter will not be addressed. There are adjusting collars on the shift signal valves. They will not have enough adjustment in them to raise the shift points to match a gas engine without changing springs and/or valves.
Done properly, you will end up with a powertrain that performs properly and stays cool. Short cutting this process can result in any of a myriad of problems ranging from annoying to potentially serious.