1958 Chevy Truck Spatan 100

Specs. 2 1/2 ton Car Hauler with a 14 ft bed
The "Spartan" name was first used on the 1958 Heavy Duty trucks, Apache is Light Duty and Viking is Medium Duty. The 1958/59 truck can be easily identified by there dual headlights.
The model numbers were also shortened starting in '58, so your Model 100 is the same as a 1956/57 Model 10000 or 2-1/2 ton Heavy Duty.
The only engine option for the 1958/59 100 Series trucks was the 348 V8 rated at 230 hp with 335 ft lbs of torque. This engine shares the same bellhousing bolt pattern as all other Chevy V8's so droping a 350 in shouldn't be a problem.
The 100 Series is a conventional Cab.
Your truck has a Spicer model #3152 or #3152A (close-ratio). Both of these have a 1:1 5th gear ratio.