1986 K10 Cutting off When Put in Gear

[lbrown]

Hey y'all recently got a 86' k10 with the 350 700r4 and I've been having problems with it cutting off when put in gear recently. When I go to put it in any gear it shuts off immediately like I'm turning the key off, so far I have replaced fuel lines, mechanical fuel pump, new edelbrock 600cfm carb, properly adjusted the throttle cable, tv cable and the air/fuel mixture on the carb. I've checked most (don't want to say all) the vacuum lines and most of the wiring to ensure everything is getting proper power and vacuum lines are holding pressure. I've tried the NSS by replacing it and adjusting the original one to no avail (I have since put the original NSS back one). Truck starts up and Idles like a dream otherwise.
Issues I do know is the heater core is possibly bad, and had problems with the break light switch in the past.
As of today, the truck was going into gear and driving fine, parked it at the parts store where the problem arouse again and would cut off when placed into gear. Had it pulled to my house where it went back to going into gear like normal. Cut the truck off and when cut on if would not go into gear again. I am now stumped and lost.
I am seeing if anyone has a fix for this.

Forgive me if this is the wrong thread to post in, first time posting in a thread before.

 

[Vbb199]

how much spark advance are you running at idle.

 

[lbrown]

Off the top of my head I couldn't tell ya. When I first bought the truck I took it to a mechanic and had him do the timing and make sure the distributor was good as I do not have the best knowledge with distributors or timing yet.