FYI, I have a 350 sbc that was smoking at highway speeds and rpms at and above 2500. It was burning about 2 qts every 100 miles or so during highway travel. and every week or so during regular city driving. also, every few days i would notice the coolant getting low, but nothing was leaking onto the ground or other wise outside the engine. it was suggested that the rings were bad, cam lobes were shot, bent push rods causing the valves to stick, and blown head gasket, and leaking valve seals. since there was no oil in the coolant or vice versa, i put the head gasket on the back burner, and since i only had about 6k miles on a brand new crate motor, i also looked past the rings (even though i knew it could still be a possibilty, i had a feeling it was sothing simple) also, it was not smoking at start up or idle so i looked past valve seals as well. i decide to start small, I thought about what i had done to the engine before the problem occured. It turns out, that after bolting on a new intake manifold a few months back, i missed a few very small piece of the old gasket along the left bank, it was enough to keep the gasket from sealing on the #7 #5 and #3 intake port, and caused it to suck oil/coolant in under the gasket. this also caused the engine to run too hot causing a white ash like build as well on the plugs that were not burning oil. even the plugs on the right bank, that were not burning oil or coolant had build up caused by the hot teperatures, even though it wasnt overheating. the plugs that were burning oil had a very crusty black build up, and caused a condition know as bridge gap, which of course leads to a severe mis-fire and lack of power, even if a mis-fire isnt noticed. after cleaning up the face of the intake ports, and carfully reaplying the gaskets, and changing the fouled plugs, cleaning the ash build up plugs and regapping them, the problem was solved. I am posting this to inform those having similar problems, to re-evaluate their work, as this was a very simple yet serious overlook on an otherwise easy job. be sure the check all of your plugs and recognise the symptoms associated with the type of build up/color of your plugs. this link can also be very helpful when determining spark plug conditions.
let me know if it was helpful.