do you have an older 350, or is it the newer 87 up 350?

I think you said your getting fire at the plugs sometimes?
what color is the spark?
should be blue spark.

when setting the timing marks on the cam and crank did you set the cam mark at 6 and the crank at 12 o clock?

did the crank sprocket have more than one groove for the keyway?

looking from the passenger side of the truck what side of the vehicle is the distributor cap's batt and tach teminal connector is pointing to?

where are you getting the power to run the distributor's coil from?

if the lifters are all down, you will need to build them up. if they were down when you installed the pushrods and rockers. you may have damaged them if you tightened the rockers to far .

and are you timing to plugs starting on the drivers side of the engine and the first plug on that side is 1,3,5,7 ansd passenger side is 2,4,6,8. and the distributor starts with 1,8,4,3,6,5,7,2, and the rotor must be pointing at exactly the 1 on the cap?

did you know that there are 2 cycles the engine makes before you get top dead center on the compression stroke?

dont just go by the damper. go by the number 1 cylinder and where its at in its cycle.