If it's wired like original, there will be a main wire coming off the battery post of the starter. Disconnect it. That will remove all 12V connections except the battery cable. Then temporarily run a jumper wire from the battery to the ignition resistor (needed for 12 volts conversion). A truck starter doesn't have a solenoid so all that's left to do is actuate the foot linkage (stomper) on the floorboard. If it doesn't run then the problem is most likely not the turn signal or any other wiring.

It sounds easy to do but I'm not as flexible as I used to be leaning over a fender to get down to the starter. The first thing I would do is disconnect the battery for safety. Then crawl underneath the truck and disconnect the main wire.

An easier option is to buy an inline spark tester to go between the spark plug and wire to see if you have voltage to the plugs. Unfortunately it will require someone to operate the starter while you watch the tester.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."