360 won't idle smooth...can't figure why

Hi gang, normally people are asking me these questions but this one has me stumped. Hopefully someone here has run across this problem. Vehicle is a '91 W250 automatic with the above mentioned 360 throttle body injected. Engine has good compression and tuneup parts but the RH inner two cylinders (4 and 6) aren't doing anything and the LH outers (1 and 7)aren't pulling their weight either. You can pull off both inner plug wires and there is no change in RPM. Throttle body is spitting normally (in fact, I swapped another one onto to it just to be sure and there was no change), EGR is working good with good gasket, PVC valve and system is working properly, Valves are going up and down normally, and no vacuum leaks that I can find. Dribbling a little fuel out of a primer can doesn't improve the idle so that confirms to me that there is no vacuum leak. I haven't had it on a smog machine yet to see what the readings are so no input there. Since there is only one O2 sensor (in the LH exhaust manifold) I figured that wouldn't be the problem since it can't be cylinder selective. The truck stays cool and runs fine out on the road and is not sluggish on acceleration. I'm kinda thinking that the exhaust crossover on the RH side is cracked and leaking into the intake runner on those two cylinders. Has anyone seen a throttle body intake crack like that? It's a bit of a job to pull and change with all the plumbing and being 5 feet at the top of the fenders so I'm hoping it's something else. Even if I had it off, there isn't an easy way to tell if it's cracked since that area is buried inside. It's a dual plane intake so you can't see the area in question from the top as that section is the lower level and under the upper level. If it's cracked, what would be different on the smog readings over one that isn't cracked? I'd hate to go to the trouble of changing it and find that wasn't the problem. If anyone has had a carb engine with one side of the idle circuit plugged up, that is the closest symptom to match this problem....acts like it's idling on 4+ cylinders. Any ideas would be appreciated.
Thanks, Mark