The transmission isn't what would limit your highway speeds, it is the rear end ratio. The easiest way out is to keep the closed drive and install 3.55 gears, available from many vendors.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.