The tach jumping around and the engine spark timing acting up points to an ignition pickup problem or a loose connection or bad ground in the system.
It could be anything from the distributor to the MSD 6AL. It is NOT beyond the 6AL, since the tach signal comes from the 6AL internal trigger signal. So it is power, ground, the distributor, the 6AL, or a wire between the 6AL and the distributor.
If you have something less than 7000-7500 RPM and 700 HP, the OEM ignition is actually a much more reliable and better choice. The 6AL is too big for anything driven on the street regularly. It will erode the plug tips, the distributor cap contacts, and the rotor. It is bigger than needed for almost all race applications. I'm at 1200 HP and 8K RPM and only use a small CD ignition box. I used OEM parts (with a MSD coil) up to 1000 HP and 25 psi boost.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)