If you have a timing light for 12 volts, just use you modern car battery and clip the timing light probe to your truck. The light doesn't care where it gets it voltage from.
The main thing with setting timing and carburetors is each engine is different. Don't be afraid to tinker with the settings and don't get hung up on numbers. George has a very good suggestion, advance the timing a little till the engine speeds up, then adjust the carburetor idle speed back down. Next, adjust the mixture screw to smooth out the idle. You may have to go around couple of times till it feels right. Once set and running, verify by driving, then check with a timing light so you know how to reset it in the future.
Modern gas is much better then when the truck was new, so the settings are different, even your location can make a difference due to fuel formulas.
Just so you know, there are two types of vacuum ports on the carburetor.
1) Full time vacuum, this is direct manifold vacuum under the throttle blade. It's always there as long as the throttle is partially close. Even at full throttle you will have some vacuum ( very little, but some ).
2) Ported vacuum, this above the throttle blade. It will only be present when the throttle is open enough to expose the port to the air rushing through the venturi of the carburetor.
If you have vacuum at a ported vacuum line at idle, then you either have bad gaskets in the carburetor, mismatched carburetor parts, or the throttle blades are open to far.
