I am looking for the firing diagram for a 1979 Dodge Ram pickup. Changed the sparkplugs and wires, but wires must be crossed because it is not running good.

Did you use the same brand and plug number that came with the truck or did you use some other cross-referenced plug?

Verify all plug wire secure, may be no problem, have heard problems with cross referenced plugs. DodgeTalk.com and click forums, If Firing order not found yet.
Besides knowing the firing order, you need to know what cylinder number is which cylinder, the rotation of the distributor(pull cap off and crank engine, note direction,
Lastly you need to know, which is #1 on the cap, usually identified.
If not identified, Turn crank until timing mark is at Top Dead Center. Note where rotor
Is pointing. On 8 cylinder it will be pointing to #1 or #5 cylinder. Either pull plug and find #1 compression stroke. Or try it one way,if it doesn't work cross the 4 pair of wires.
It will be one or the other. Good Luck. What size Motor?