Inliners International Forums General Discussion Engines 250 chevy timing

The 250 in my truck has recently begun running sluggishly as though a couple of cylinders aren't firing. I get occasional intake backfiring. I have pulled the plugs and cleaned them. I put the timing at around 8 BTDC. At that timing I get around 12 inches of vacuum, and the engine is on the verge of stalling. I tried setting the timing with a vacuum gauge to the point where I get maximum vac, around 18 inches, but at that point the timing is off the scale and the backfiring is more frequent. The timing is set with the vac advance plugged.
The engine will accelerate smoothly while sitting with the trans mission (T-5) in neutral. When I accelerate in gear the engine bogs. It will move down the road but it does have any spunk. Runs "flat".

Any suggestions?
Thanks.
Ken