slant six cranks but won't start

OP you haven't said a thing about what you have tested (besides replacing parts)

1...Have you tested for spark? IMPORTANT. "Rig" a test gap or overgapped plug to the coil with a WIRE core wire and not a resistor wire. Crank using the key and check for spark. Should have a nice blue snappy spark at least 3/8" and more typically 1/2" long

2...Fuel? Has this BEEN running? Or parked/ stored? Is the fuel stale? You SURE?

3...Are plugs fuel fouled?

4...Have you run a leakdown/ compression test?

Diagnostics are IMPORTANT and throwing parts at something, whether you can afford it or not is no way to "fix" something.

(Example, you "throw" a bunch of money at tuning it up---which it may or may not need, only to find something "else" in the end like a poor connection/ path from the ignition switch to the ignition system--or maybe even a bad switch.)