Help diagnosing a rough running 292

[M_R_Q]

Hello,
My truck is a 68 chevy K10 SWB with a 292 and a 4 speed. Its an old farm truck thats been in my family for years. The last time I drove it was probably 10 years ago. It was running rough, then just got worse and worse to the point that it was parked. Im not much of a mechanic but Ive been trying to get it running again, and I just cant figure out the issues.

Symptoms: Runs really rough, misses badly and coughs out the carb and backfires out the exhaust. Barely has enough power to move without stalling.

I have a good running 71 with a 250 that I stole parts off of to help find the problem.

Ive swapped the coil, distributor, points, condenser, cap and plug wires with the 250. The 250 still runs well on the 292 parts, no change on the 292. I pulled and cleaned the 292 carb. No change. I swapped on the 250s carb. No change. I pulled the valve cover and all the valves seem to be moving at the same rate. I did a compression test, and compression was all between 120-130psi which I understand is normal, especially for a motor that was fairly tired to begin with.

One thing I have not changed are the plugs. I gave them a good cleaning, but since the plugs are one thing that cant be swapped from the 250, i opted to not buy new ones, mostly since I wouldnt think the plugs would cause the backfire issue.

Could it be the timing gears? when I roll the engine over by hand, then reverse the direction, there is a bit of play (maybe 5 degrees) until the rotor starts to move.

Any other thoughts on what I should try? Can bad plugs cause a backfire/coughing issue? To me it sounds like a timing issue, but I dont have the knowledge or experience to say for sure, and Ive changed everything related to the timing except the gears.

Thanks