I dont know the answer ( maybe the advance ,distributor ) But the fact that it does it in gear and not idle should get you a good answer from someone , its a good clue.

 

Vacuum Advance - How to Test - YouTube when i mentioned (advance distributor) I went looking and found this ...after this vid you find one that says checking for leaks..guy uses propane instead of spray cleaner to indicate vacum leak.

 

a bad coil can show itself wth the symptoms you are describing...because over time the electrical shellac can break down on the secondary coil windings,which essentially reduces the length of the winding,a short cicuit...and reduces the strength of the coil.You may be able to get the motor started,but once the motor is under load the spark may be borderline,and not be strong enough to jump the gap at the plug.I had this experience with my '53 Ford 'flatty',with points ignition,of course.Your situation is not too much different,although you have 'breakerless' ignition,with an external module I believe.I had two different '79 351M Cougars,with external modules that would show failure by oozing epoxy due to overheating,although I changed them out before they quit altogether..

 

Unfortunately the only way to check a coil is by substituting another coil unless you have some additional test equip.
Ohmmeter tests aren't always dependable .

 

my reply on 9/4 12;14 PM ..The first sentence is a link...

To add info about coils , I remeber a mechanic telling me that a bad coil is most defective when engine is warmed up.

 

While you are pondering what to do next, did you check the PCV hose coming from the back of the carburetor to the PCV valve? If that hose is shot it will allow the engine to run, but when you place a load on the engine the large vacuum leak just leans out the fuel and the engine won't stay running .

 

Hey guys finally figured it out! The distributer. It was hard to tell at first cause it didn't spin and looked how it should. But my truck wont start anymore and I looked again and it just spins now. So im guessing the pin was in the process of sheering or coming out. I also looked at my spark plugs and they looked like they've been dipped in Clay, I think cause the distributer was a little off. I have one more question, I hear that if your not careful replacing the distributer the pin can call into your engine and ruin it. True? Should I have s Mechanic replace it? And thanks everyone for the help!

 

I'd be looking for a new or r/b distributor...or at the very least a good one from your local junk yard...Well,OK now,are you saying the distributor shaft/rotor turn freely once you've popped the cap off?..If so,you may be correct in thinking the distributor drive gear pin is sheared,or worse.Taking the distriburtor out carefully is your next step,but firts align your timing mark on the crank vibration damper before proceeding.After it's out,I'd use a long reach magnetic pickup to fish around the camshaft worm gear,to see what you find.
It sounds to me that this motor has not seen an easy life,or regular oil changes,and sludge has plugged the distributor lubrication galleries as well....go easy,step at a time,and look things over real well.On restart,your timing will either be correct,or 180 degrees out..you'll know! Good luck!

 

Well I replaced the distributer but it still wont start. 98% chance I just did it wrong but il be working on it all day to get this right and check some of the other things people brought up.

 

How to Replace A Distributor Without Going Crazy I know that you already changed the dist. But this may help you find out what you may have done wrong.....you said the plusgs looked like they were dipped in clay ..are they wet and gooey looking?