Causes of slow spark?

What could be a general cause for slow spark? When tested with a screwdriver, any given plug wire will fire once, maybe twice per second of cranking. Doesn't seem to change between slow-ish cranking (when battery starts to die), or faster cranking (jumping off of another car). Can go more in to detail but the basic story is this: the weather got cold, truck wouldn't start, plug wire and coil wire spark was slow and weak, changed LB dist to better vac advance dist, fixed coil wire spark but not plug wire, swapped from LB to GM HEI, plug wire spark is now mostly blue with some orange, just slow as previously described. Distributor timing is unknown at the moment, and probably out of wack, i've had it in positions where it hits well and wants to start, and spots in the opposite, still the same spark speed as before anything was touched.

Something else i'll add while i'm here, my old lean burn dist had 4 sections at the bottom where adjustments could be made, 90 degrees or so at a time, a local buddy of mine said that the vacuum can on the other dist should be pointing at cyl #2 or around that area, as apposed to where the lb dist was (can would have been pointed around cyl #8, if it had one), but the vac dist only has three of these 90 degree squares at the bottom, the missing area being the one i'd need for what general area he is talking about.

It does have a brand new coil, distributor and cap are clean, HEI is wired up correctly, plug wires are just a few months old (granted, they are duralast), plugs are good, and vac advance is hooked up to the correct carb port. It hasn't jumped time (tested cyl 1 at TDC). The most weird thing about this, is that when I had the vac dist in the ballpark where the LB one was, after letting it sit from cranking for a couple minutes, and cranking again with no pedal/ gas, it would almost fire right up, then go back to cranking. Happened consistently that way, regardless of smaller adjustments made to dist timing, and especially without giving the carb gas. Stumped and confused (lol), any help is greatly appreciated