Need more info on your transmission to identify the ratios of each gear, particularly first gear.
Is the transmission the 3 speed that was original to the 1946 truck?
However, the info you’ve provided should be enough to calculate top gear. With a 3.38 rear end ratio in 3rd gear (1:1), and your tires which are 26.9 inches in diameter, you’d be cruising 50 mph at roughly 2100 rpm.
