I bypassed the filter for the Tach, still reads way too high. It's a 86 305 4spd auto all stock.

The Tach used to show 2100rpms @ 75mph, now it's off the scale.

Any advice is appreciated.

I don't think it will work without the filter. Too much noise from the HEI. 2100 @75 sounds about right if you have low gears ( like 2.23). I'd say put the filter back on, and then we can go from there if there is still problems to figure out.