you need to get that lost bolt out of there. take a light and shine it down an ajacent hole while looking down the hole you think it went into. it should be hung up against a lifter or something. you will need a magnet. try not to send it further down.
have you done ay of these checks yet:
-cooling system pressure test, at the pressure that your rad cap is set for. don'toverpressure the system or you could blow a hole in the rad, heater core or a gasket somewhere. also, check the under side of the oil filler cap for a white milky substance. that would indicate a leak somewhere from the coolant into the engine oil, condensation from that collects on the under side of things like the oil filler cap and valve covers, but may not show in the oil on th dipstick yet.
-check the spark plugs for a rusty looking one, like if that cylinder has a cracked head or leaky head gasket, it would leave that spark plug looking rusty.
-check for bubbles in the rad when the engine is running, possible head gasket leak.
- a timing chain slack test, turn the engine over by hand till you come to the timing marks. take the ditributor cap off and make a detailed note on where the rotor is pointing. use some tape on the edge of the distributor housing. also note where the timing marks are pointing to on the harmonic balancer. now turn the engine backwards slowly until the rotor just starts to turn and "catches up" to the crank (from slack in the timing chain). the number of degrees the harmonic balancer moved before the rotor caught up is a good indicator of how loose the chain is. a new engine would basically be zero. some of those old sbc had plastic covered timing geras and the plastic would wear or break off. instant loose chain. they can skip over a tooth pretty easily, causing the cam to be out of time with the crank.
-vacuum check at idle. does the needle skip around a lot or hold pretty steady?
-valve adjustment. include looking for broken valve springs and also check to see if all the valves are opening the same amount. some of the older sbc had soft cams and the lobes would wear off.
-cylinder leak down test. air up the cylinders at tdc ready to fire, see if air leaks past the valves down into exhaust or up into intake. checks for leaking or burnt valves.
-compression test, wet and dry. dry first for obvious reasons.
-carb adjustment, including check to see if the throttle shafts are worn out. also a choke set up.
-heat riser valve check.
-egr valve check, if equipped.
-pcv valve check
-vacuum hose check. inccluding the one that goes to the auto trans modulator (if equipped).
-distributor mechanical and vacuum advance check. include check of wear on the mechanical advance weights and springs and freedom of movement. also base and total advance. vacuum advance hose routing from proper connection at carb and (if equipped) does the vacuum temp switch operate properly to allow vacuum to the distributor when the engine warms up.
-condition of the spark plug wires and distributor cap, rotor, coil, plugs. rest of tune up parts like fuel filters, air filter, pcv valve.
-adjust timing correctly at operating temp.
hopefully that helped. sometimes a problem like yours is a combination of a bunch of things being out of whack, or it's something that was overlooked. go through the list and see if all is good.
on the valve cover leak, what works for me is to flip the valve cover upside down with the gasket surface on the edge of your workbench or something flat. the rest of the cover has to be supported by a buddy (or your belly or knee) because it will be hanging out in mid air. take a ball pien hammer upside down so the ball is in the bolt hole of the valve cover, give the hammer a tap with a dead blow hammer and watch the bend come out of the cover. when it is all flat, make sure the surface is clean of old gasket material and then glue a new gasket to the cover and wait till the glue is cured, then install it with those little butterfly washers, or get some longer ones at a speed shop. I have seen some that are couple of inches long, they work well. don't overtorque the bolts or you will split the cork gasket. i like to use the neoprene gaskets for that reason. felpro sells a good set but i don't remember the number.
good luck and keep us posted,