Hey sambo, thanks for following.
The N/A version of the FC RX-7 in the US actually came with a down-specced brake/wheel configuration. For the 1986-88 N/A FC in the US, instead of a 5 bolt pattern, it used the same 4x114.3 wheel pattern, 14" alloy wheels and the same size single piston front brake calipers and 250mmx22mm vented brake rotors that came on the series 3. Unlike the integrated hub on the series 3, however, it used a rotor over hub design. It also used nice and light alloy hubs with 4 lugs instead of 5.

You need series 4, not series 5. Series 5 N/A US models still had the smaller brakes but switched to the normal 5x114.3 bolt pattern.
So armed with that knowledge, I located this pair of the alloy 86-88 N/A FC hubs from the US:

- 20151202_184259.jpg (191.26 KiB) Viewed 5168 times
The hubs will be milled to make the bearing replaceable. A spacer will be needed to offset the FD brake rotor being used and allow the brakes to fit with my wheel combo. I will also be installing ARP extended heavy duty wheel studs.
As for track, it is true that the FC overall track is greater by 30mm overall (ie 15mm each side), but the body dimensions and strut top location on the 1st gen are also different, so the FC track measurement becomes somewhat theoretical - everything changes. Plus the guard clearances are different too. When this mod has been done in practice, the hubs end up in basically the same spot as a 1st gen.
The main practical consideration is really brake and wheel fitment. The combo I've chosen has worked for others, but if there are any fitment issues, these can be addressed with the hub spacer if too narrow, or if too wide I'm assured ACBron has ways of fixing it.
Track can also be manipulated through playing with wheel offset. But in my case, I've already got 17x7 wheels with custom offset, which I'm keeping. My current offset is such that my track is probably narrower than factory right now. There's lots of room under the guards even if the steering and brake conversion pushes out the wheels slightly (see below). Plus with the adjustable strut tops, I easily have another 2-3 deg of negative camber I can dial in.
Rear track I'll be leaving alone, but pictured above I have 17x8 wheels with 235 tyres.