New engine + new tranny = same top speed! (Unless that tranny has an overdrive...)
Top gearing on most any transmission is 1:1, meaning for every turn of the engine the driveshaft turns once. If you swap the engine and tranny, the 1:1 ratio stays the same.
An overdrive transmission drives the driveshaft faster than the engine, so that is an option. I don't know if the T5 is such a tranny...?
Changing the "pumpkin" as suggested can also improve things. The 3/4 tons are geared slower to haul heavier loads, but with a lower top speed.
-Boyo
