I am hesitating with choosing of intake manifold. 1966 HT Stang, 302 Mexican block, flat top wiseco pistons decked, 298 home ported ( cleaned ) heads (10:1), XE268H cam, long headers, 4-speed manual tranny. Performer RPM or Performer RPM airgap ?? Does it fit under the bonnet ??
While some argue that there isnt' much difference between the two, dyno tests continue to prove that the Air-Gap does make more power. I have one on my 302 and love it. It does fit under a stock hood.
I have the air-gap and it fits under my hood (i have a drop base air cleaner 14x3)
You will be VERY happy with it, performence wise AND the way it looks.. Im still awating to choose my cam, im thinking the 280H and some head work while its apart will really make good use of the intake ( but it seems you have allready done that) so i vote for the Air-gap
i've got one on a 351w but it wont fit under a stock hood with an aircleaner. however it looks good and it has an excellent throttle response. its better than the regular rpm because the air gap gives it a cooler denser air flow charge into the cylinder.
The 351W is about a half inch taller than your 289 so you should be fine. I haven't installed mine yet, but based on reading a half dozen intake dyno tests, it seems to be the best of both worlds: it wins on the bottom end compared to dual plane intakes and just about matches a single plane on the top end.