[rpmerf]

The timing should be less advanced with a load. There are 2 main things that effect what the timing should be - RPM and fuel in the cylinder.

As RPM goes higher, you need to ignite the fuel sooner, so it is pushing down at the correct time. In a mechanical distributor, this is handled by the mechanical advance.

As the load goes up, there is more air and fuel in the cylinders. This ignites faster, so less advance is needed. On a mechanical distributor, this is handled by the vacuum advance. Typically adding around 10* at high vacuum, and 0 at WOT.

Some numbers from my truck with points. These seem to be somewhat standard for a good tune on a mechanical distributor.
16* base, 18* mechanical, 10* vacuum, all in @ 2500 RPM.

At idle, it will see 26* (base + vacuum).
On the torque converter, it should see around 20-25. Depends on the stall speed and how hard you are on the gas. (base + a couple degrees of mechanical and vacuum)
Cruise around 2k RPM, around 36* (base + some mechanical + vacuum)
Cruise around 2500 RPM, 44* (base + mechanical + vacuum)
WOT @ 2500 RPM, 34* (base + mechanical)

I would imagine, in the end, your numbers should be similar. If you converted your engine to carb and threw and HEI on there, this is about what you would be tuning for. I would be interested to see what a stock truck has for timing. It might even report in the scan tool what it is expecting.