So not a Volvo box?


What Paul's referring to is the inertia effect. Any of us who have been playing with mechanical bits have seen the huge inertia that builds in large step down gearing- a lathe or whatever.
You have that effect in the drivetrain. In fact it's been said that reducing a flywheels weight can be like adding 40lbft in 1st gear, 20 in second, and essentially undetectable in increased performance on the upper gears.
Any time you use a numerically higher ratio, your drivetrain is further fighting it's own resistance to gain speed quickly.

If your conversion to another box caused you to use a heavier than stock flywheel, I would look there first.