I always say going to MegaSquirt for a single feature is rarely cost effective, but the more features you use the more cost effective it becomes versus the cost of piece-mealing different features.
You'd have to upgrade the whole fuel system to a high pressure system. And most aftermarket fuel pressure regulators for EFI are by-pass return type with a port for vacuum/boost reference. Then the cost of the injectors, fabrication of the fuel rail if no one makes one, etc.
To speak just of the ignition system. You'd probably be about $500 in for the MS3 and knock module and wiring. Now that I think about it, I actually wonder how the J&S system works on identifying the cylinder? Does it come with a dual sync distributor to identify cylinder #1? I don't think the MegaSquirt will identify which cylinder to trim timing on without crank and cam sensors, but it can do global timing retarding on knock with just the simple tach signal.
IMO, just for the cost J&S box it sounds like the Mega Squirt could get you some pretty good ignition controls that will adjust for boost as well as do rev limiting, 2-step rev limiting, shift light controlling, high boost warning light, datalogging of AFR's, rpm, timing and boost etc. There are a dozen input and outputs you can utilize. All while leaving the carb/fuel system in place. The fully assembled MS3 is $559 plus $85 for the knock module, plut $140 in a ready made harness that you just have to shorten the wires and connect to the sensor plug. So it adds up to quite a bit more than the J&S box all in.
The only wires you'd need are the power, ground, tach signal (from points or pickup coil), ignition out to control the ignition coil, your existing O2 sensor in, knock sensor in and maybe a coolant and intake air temp sensors for data logging and if you want to adjust the timing based on coolant or intake air temp. Then a vaccum/boost line to the MegaSquirt. That's 8 wires plus a vacuum line. Well, maybe up to 12 wires as the sensors will want ground wires too. If you do this bare minimum you can probably just use an MS2 and an aftermarket knock sensor board. $435 fully assembled plus $67 wire harness and I'd have to dig up the external knock module. The MS2 does essentially the same thing as the MS3 minus full sequential fuel and spark control. More cost competetiveness and can be upgraded to an MS3 down the road if you decide to take your project in that direction. MS2's batch fire or semi-sequential works perfectly fine, especially when compared to a carb.
Just food for thought. I know it's not for everyone.
