"1949 Chev 3600 truck with a tired 235"
A tired 235 can mean an engine with worn rings and lost power/torque. If you are often at full throttle at 45 mph then yes, more power/torque will increase top end. You need the power/torque to overcome wind resistance and haul the weight of the truck and occupants. But there is a point (Jerry's comment) where the torque of even a rebuilt good engine will start declining on the RPM/torque curve. The analogy is a team of horses pulling a light load can only run so fast. Adding another horse doesn't mean it will run faster.
A 1949 will have a torque tube drivetrain. Upgrading transmissions or rear axles means converting to an open drive shaft which involves $$$. If you want to drive your truck often on the expressway then yes, I vote upgrade. You also should think about upgrading your braking system (bettering stopping power) and do all you can to the original steering to make it perform optimum. There are crazy drivers on our expressways today so you and truck may need to react quickly.
Changing to taller tires is cost effective. You may have to change the gearing for the speedometer.
