350 Chevy Van slows down + pings on hills

Timing could be an issue, certainly.

But the first thing I would check would be the spark plugs. How long has it been since you changed them?

If the plugs are fouled ( typically with white combustion residue), or if the electrodes are thin or opened up beyond spec, or if you have inadvertently installed the wrong plugs, you can get this sort of behavior.

Check your timing and advance (I assume it is adjustable on a car of that age) while you are at it.

It could be loaded with carbon, initial timing too soon, old enough it may have one of the spring and weight kits that advance your timing faster installed, burning oil, I can think of more.

Timing. Dirty combustion chambers. Mixture too lean.

and a few other things.

I'd start with timing, test the distributor for proper operation (vacuum advance)then move to other things.

How many miles on the engine (since last rebuild, if approprirate).