1986 GMC Sierra 3500 2wd

[77GMCSerria]

I'm looking at this 1986 GMC Sierra 3500 2wd Dually(at least that what the ad says), from what I have researched this may not be a factory 1 ton dually:

In the description they state that the engine is a v6 292:

1. From my research GM never made a 292 v6, but they did make a 4.3L 262 v6.

2. In 1986 the 262 v6 wasn't available on a one ton truck but the 292 L6 was an option.

If in fact this is a 4.3L v6 it seems that this would be a weak engine for a one ton dually. What would be a good easy replacement engine?

I'm guessing that this truck has a chassis from a 3/4 ton truck and a body from a one ton. I have not looked at this truck in person yet and don't have access to the VIN to decode it. What is the diffrence between a 3/4 ton and a 1 ton (besides the suspension)?