I know that European brochures usually showed more technical information, but the 3.1 from GM would have hardly been a European spec engine.
An approximation method may be to look at the V-8 engine with similar bore and stroke from the same vintage and proportionately scale the HP and torque curves. I'm pretty sure that the truck engines have a greater following that the 3.1s.
Maybe looking at the service manual too? There may be a diagram in the back. (Long shot.) Or searching through the road tests of old car magazines?
Good luck!