Inline 6 is really tired

[AussieinNC]

Chevrolet 292 engines achieved a maximum 165 gross-horsepower at 3,800 rpm and netted 147 horsepower at 3,600 rpm.

It reached its maximum gross torque of 280 pound-feet at 1,600 rpm and netted 262 pound-feet of torque at 2,000 rpm.

Your engine with the 5th gear ratio of either .73 or .80 is right around the torque net range value of 2040 rpm (.73) or 2240 if using the .80 version of the T5.

These trucks were fitted with 4.11 rears to accommodate the relatively low torque figures of the six vs the V8....

Add in the age of the engine, the weight of the truck and its high level of drag...

A 4.11 rear gear equates to a 3.00 rear gear if using the .73 version of the T5
A 4.11 rear gear equates to 3.28 if using the .80 version of the T5.

You have a "normal" aged six cylinder truck....

A swap to a 350 and auto will improve the performance....