[ZoomZoomMX-5]

I've built a number of 911 EM's and have seen most of them.

Unfortunately you're going to have to buy all three kits to do this car unless you want to so some creative scratchbuilding to modify some parts, and you'll still need to come up w/foglamps unless you also buy the '73 rally car. There's not enough shared parts in any of them to accomplish what you're looking for w/o getting all three kits. You will have a treasure trove of spare parts after the build.

I'd use the 911R as the starting point and add the 73 RS front bumper/corner lamps. I'm unsure how visually distinctive the 74 3.0 engine is vs. what's in the others, but the kit has rollbar, racing seats etc.

From the spares you could build the '74 RS 3.0 to depict what Porsche sent to the US for the IROC cars; they were delivered w/the '73 RS ducktail from the factory before being replaced stateside by a longer-than-factory whale tail. You can use the headers from the 911R, perhaps the intake trumpets too.