Changing rear end ratio

[RatRacer]

'78 G30 van, 400 small block, TH350, 14 bolt rear end.

Gearing is 4.10, fine back when the national speed limit was 55, not so good going 80 on the highways around here.

I'm assuming there are different gear sets made for the old 14-bolt 10-1/2" rear end, and that I can somehow figure out which speedometer gear to change it to but what about the governor in the TH350?

I'm familiar with changing the weighs and springs but I can only find kits to raise the shift points, not lower them.

And what rear end ratio would you suggest? It's basically an around town van with city and highway trips, only occasional cargo - nothing heavy and no towing.