If it were mine, I would add another gear like you thought it needed. Either a 5 speed overdrive or a 700R automatic. You'll keep low end torque and gain more top end speed. As far as modern upgrades, it would be easier to modify your chassis with kits and conversions that a complete body swap to a different drivetrain.
That's my 2 cents,
Scott


Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!