Adding my 7.3 cents to bbjordan’s 6.9 cents …
Timing is everything on these trucks so you need to eliminate that from the equation.
Take your Gunson out and try the following.(I am guessing you did most of this but rather state the obvious than assume you did.)
1) Make sure you place it on the front passenger side injector line closet to the radiator. Sand the paint off the injector line under the piezo clamp to insure a better read. Often folks have a issue trying to read next to a bend in the line. So you may have to move it after the bend. But keep the clamp as close to the injector as possible. (0-6 inches.)
2) Keep your Gunson box as far as possible from interference like the alternator, heater box fan, radiator fan, etc….. This might be causing the box or timing light to be confused between different signals.
3) Hook up your timing light. Note that not all timing lights like these style of induction rings. If you have a buddy with a different brand or model of timing light borrow it if yours is not working. But try yours with the induction clip one way then flip it over the other way.
4) Still no flash on the timing light flip the injector line clamp or move it. Maybe put a little needle in between to make it clamp harder.
5) So you have a flash but -
a) It’s flashing erraticly, that is ok go for it, likely it is missing beats/injector pulses but not showing erratic timing.
b) It starts flashing erratically at 2,000rpm, go for it, likely missing beats/pulses but not showing erratic timing.
c) It’s the timing light is flashing but not showing the timing mark or so far off you can’t adjust the IP over enough to get on time. Then see comment below, perhaps IP is installed wrong.(IP is so far off (180*), the timing mark is out of sight When the injector fires.)
I have heard folks put the IP on 180* off, then have issues similar to yours. (Lack power, white/grey smoke, idles, but runs poor, etc.). So don’t rule out the pump could not be on right.
Hopefully you can get your timing tools to work and take that out of this equation.
Send some more pics of your Gunson and timing light.