No modifications are necessary to run 4 injectors from a TBI PCM. I have a friend with two cars running stock (unmodified) '7060s, running home made 4 bbl throttle bodies made from pairs of GM throttle bodies. He simply wired the injectors in series pairs and has had ZERO trouble for years this way.

I agrre a single ECM solution is best.

Also progressive is not easily do-able with a stock GM ECM, since it has no provisions for progressive secondary injectors. If that's a must or highly desired a Megasquirt is what I'd recommend. The problem with trying to run progressive throttles without also running progressive injectors is that you will get a large amount of fuel puddling above the secondary throttle plates and then when they do open will cause a rich condition. This will not always happen at the same load or RPM, so it would be difficult to tune around, also if you stab the throttle let off and then right back into the throttle, you'd likely not get that excess fuel on the second stab, so that would cause a lean condition. Best to just keep the throttle linkage 1:1 for all throttle plates if you go with an ECM that's not capable of progressive injector control.

As a side note, I like the crossfire system. I have on eon my grandfather's '71 truck, running off a '7427 PCM, that is also controlling a 4L60E. :)