if your truck was running okay before then I would double check the work that you did.
Gap the plugs at .035"
Make sure the spark plug
wires are on the correct towers on the dist.
When you changed the cap did you take the ground strap from the old cap and put it in the new cap? It goes from one of the four screws that hold the coil in to the center pin where the electrical
connector plugs into the Distributor cap.
There is no need to change the distributor timing so that should still be the same.
Let me know if it is not one of those things and I can tell you what you may need to check next.