My Vortec had similar symptoms when I bought it. It idled rather rough only when in gear. I thought for sure it was a bad spider assy. I bought it reasonably, so I decided to take a chance on it. I drove it a month or so before really diving in and diagnosing the rough idle.
What I found was one cylinder that had ~60psi and the rest had ~150-170psi compression. I also found that someone had ratcheted up the throttle cable so it would run in gear (found this when got it running after rebuild - as it had a high idle). When I pulled the engine apart, I found both cylinder heads were badly cracked between the inner cylinders. All 8 combustion chambers were a bluefish color (extreme heat had occurred at some time). It also had a bent connecting rod. The engine had what appeared to be new intake gaskets, new water pump, new radiator, new plugs/wires/cap/rotor, and new egr valve.
It looked like someone was just throwing parts at it, without checking the elementary stuff first. I rebuilt it with one 'seasoned' connecting rod, new sealed power pistons, all new quality components (Hastings rings, clevite bearings, melling, cloyes, etc), machine work, etc. it now has about 7k miles on the rebuild and has run great since. I did all the work myself (except machine work) and had less than $1000 in the entire job. SBCs are great that they are so cheap to rebuild. I now have roughly the same $1000 in a set of pistons for my Pontiac 455 that I'm building now.
Guess I'll have to ride 45 min to the next nearest parts place. I been working till after 10 at night and haven't had time.
