The brakes aren't dragging are they?
You say it revs up fine in neutral/park but not down the track.
heres a few more suggestions.
Timing curve may be way off, or not enough advance.
Weak spark, lack of fuel E.G. not enough fuel pressure, improper fuel bowl levels, under or over jetted.
Sounds like your making some headway with the ignition, that shoudl help.
I've seen old or just plane wrong plugs/improper plug gaps cause poor performance.
Try some NGK's heat range #8 gapped no wider than .030
Keep pluging away.
Got any video's of the car going down the track?