Next time you have the Q-jet apart, put some epoxy around the primary and secondary fuel-well plugs on the underside of the main carb body. File around the edges of the plugs to get clean metal for the epoxy to grip onto. These plugs frequently leak fuel into the intake, causing hard starting/flooding after sitting for a short time while hot. Also allows the fuel to drain out overnight and the engine needs a few cranks to get fuel back into the carb. If the engine does fire right away when cold, the problem could be a needle valve not seating and fuel overflowing into the intake, which matches your complaint of lots of black smoke. Seal the plugs anyway. This is likely the source of your grey smoke on startup, and should also take care of your hot start/ 1/4 throttle problem.
Bad guides give a puff of blue on startup and at an idle, and it sounds like they were checked, and new seals should take care of that. A leaking vacuum modulator on the trans will allow tranny fluid to be sucked into the intake via the vacuum line (back of the intake manifold).
Once you get the carb back on, you can go through the tuning list found at Quadrajet question. How well is your distributor set up? Regarding tuning the carb and distributor for power, a friend of mine was having trouble with his Buick 455 not feeling quite right. Swapped out his carb and dist. with mine, and went from about 5 feet of rubber to omigoodnesswheredthecargo? Doubt if your car is that badly out, but it should put down some serious black.
Q's point about the cam timing is important. Most street cams benefit from being about 4 degrees advanced. I also don't trust the timing set to be accurate and check every one. If the cam is retarded, you'll be down a bit on the low end and may not make it up on the top end like most people think. The worst I've seen (other than having the timing out a tooth - oops!) is a cam that had the keyway out enough that it was halfway between teeth on the gear - had to build an adjustable gear for it (OHC furd)!
To check your cam timing quickly, remove the left side valve cover, remove the #1 rockers and spark plug and get the #1 on TDC with the pushrods both moving (exhaust closing, intake opening). Reach into the cylinder with a narrow screwdriver and rotate the engine to TDC several times (rock back at least 20 degrees each time)to verify TDC on the timing tab (only count the normal direction of engine rotation when noting this). Be as accurate as you can. A long dial indicater is best. Bring the engine to TDC, rotating it in the proper direction for the final time. Put a straight edge across the tips of the pushrods. The intake should be even or slightly higher than the exhaust pushrod. .050" higher is about 4 degrees advanced. If the exhaust is higher (double-check), you may want to go back in and correct this. Like I said, this is a bit of a rough check but works.