If you don't have a MAP sensor input it can't work. There is no other way to do it safely. If you move up to a Grid you can add a boost control module that also does timing vs boost. However if you are willing to go that far you should consider a Haltech VMS/VMS-T which can use your existing 6AL and have full control of timing, boost, data logging, COP/CNP ignition, etc. Check out the Haltech website. Its an amazing unit for the money.
I was thinking that you could use 2 of the retard features wiring the trigger wires to a Hobbs switch that is normally closed.since you have to remove grd from trigger wire to activate. This will be a blow thru carb deal, pump gas with 6 or 8 lb on the street.