There's only three ways you can increase the top speed, change the differential gaering, install an overdrive, or put larger diameter tires on. None are going to be inexpensive to do. Changing the differential gear may be the hardest without changing out the entire rear, even then, rears of that capacity tend to be very low geared. Best comprimise there might be to swap to the optional 2 speed, which has a 6.40:1 high gear.
A trans swap will be easier, as there were several overdrive tranmsissions made for the heavier trucks, Fuller, Clark, and New Process all made them, plus you could possibly even use the NV4500 5 speed from the later GM and Dodge trucks. There's also the option of using a Ranger Overdrive from Advanced Adapters, which would bolt in place between the stock trans and bellhousing, effectively giving you 8 forward gears.
A note on the wheels, don't weld them! The multi peice wheels reguire disassembly in order install tires on them. A better choice would be to seek out the 22.5 single peice wheels from the '56 and later.
Now for some inspiration. This is
Grant Ensrud's '56 6400, which is very simlar to your '55. Grant uses this truck in his landscaping busines, it gets worked pretty hard. The description is a little old, the 496 is in it now, and the Fuller gave way to an Allison 5 speed automatic.
