International 8100 trans upgrade ideas?

Hi, I have a 95 IH 8100 tandem DT466 dumpster truck. It originally had the 6+1 spicer but was changed to an Allison MD3060 auto. The MD has been rebuilt at least twice in the past couple years. I have a feeling it's far too weak for our application.

Does anyone have a suggestion for a non-computer auto trans for this application? It has the Spicer N340 axle 34000 5.63. It doesn't need to go over 55mph.

I appreciate any thoughts.

 

An update... My mechanic informed me that the truck actually has the MT653, not the 3060. To my knowledge full synthetic has not been used. This truck has no trans temp gauge. It looks like the shift cable has a lot of slack, we're going to try to tighten that up and see if that improves things.