I did the math and decided it wouldn't be worth the cost and effort. It would still be close to over reving at 55.
Stock overdrive is 0.79
0.79 x 6.72differential = 5.308
Allison, for example 3560, overdrive is 0.65
0.65 x 6.72 = 4.368
2600 RPM's/5.308 = 489.8 axle turns per minute = 56mph stock tires
2600 RPM's/4.368 = 595.2 axle turns per minute = 68mph stock tires
Add taller tires & you are comfortably cruising the engine well within safety rpm's at 60mph.
Even the A2's with 0.75 OD would see an increase in top redline speed to about 65 mph using stock tires.
But if you are going to do a tranny swap, why keep the multifuel engine? A Cummins 5.9 would give you more hp, more torque, higher revs & readily available parts. And at 500 lbs lighter it easily counters the extra weight of the Allison. My philosophy is either keep it stock, or modify it up to the limit of your budget. Tranny swaps are expensive. May as well do an engine swap while you're at it since both will require bell housing, ecu's, driveline mods etc. You can mate the new tranny up to a new engine outside the truck, then put it all in at once. It would make mating it up a heck of a lot easier.