I've not heard of the AF reed valve but I have just fitted an Atomic Ultimate one to my Avanti. I'm not sure if these are still available but the manifold fitted the barrel with a little trimming of the fins but it did also need a fair bit of grinding, repairing with plastic metal, more grinding of the top of the manifold itself to get it to fit without hitting the frame and I had to also resort to fitting offset cones. I also had to cut a couple of windows and a cut away in the skirt.
The result so far with it jetted as per my TS1 engine is very pleasing. I'm running it with a Breath Sweet filter and it pulls without hesitation or bogginess from 20mph in 4th right up to at least 65mph, performance wise it feels unchanged up to that point at least. That's on 4.8 gearing and I'm loath to push it any harder until it's been on a dyno. No noticeable leakage around the carb/filter so far (100 miles).
The LTH filter would work too but the MB one won't unfortunately.