At the risk of some sort of argument, you can actually modify the tunnel in IPRA in order to accomodate a gearbox of more suitable strength. You can't change the firewall though, that's true. I think if you were really tricky you could get one in an 86 with no firewall mods nor any swaybar mods. And this would have far more benefit than simply avoiding a custom swaybar if you get what I mean.
Also, you can run the 6 speed as plenty of IPRA cars already do, you just have to lock 1st gear out. Which is fine as its of no real use anyway. This gives you a much closer set of gears (2nd-6th) than a stock W box. If you're rich however, or at least have the budget to do things properly, get a built W box for sure.