rough idle has me stumped

[bumpster]

Aquired a 1970 GMC 350 auto,quad, automatic. Truck runs great as soon as it comes off idle, but at idle the whole truck just shudders. Thought it was a vacuum leak, but could not find one. replaced all hoses just for good measure. Then I found Champion plugs and off-brand points, so they went away. I then went into my fit to be tied mode and changed the entire ignition system over to HEI,(don't tell shuttermutt where I got the parts).With timing at 8 degrees and idle set at 950 (no load), it becomes almost bearable, but still not right. Exhaust sounds like a steady miss, but when I pull plug wires,none of them fail to drop rpm's about 75 to 100. Compression is a little low at 90, but no more than 5 to 7 lbs difference between cylinders and that was on opposite banks. Any ideas before the rest of my hair comes out?