[Johnnyboy]

After thinking about the 6 speed Allison. I was thinking you mentioned your 5 speed transmission had a transmission retarder? If that is correct, those were quite expensive and very effective. You would loose that extra braking power, I believe you would not have any engine braking with the Allison 6 speed. You would have to look at a Jake brake, which I don’t think they were as effective on the 2stroke diesel.

If a transmission retarder is what you have, you may consider keeping the transmission and possibly changing your rear differential gear, instead for a higher cruising speed if that is what you are looking for.