Claudio: The SAITO 72 will handle a large variety of props.
Most people, including me, run two-blade props on 4-cycle engines but I have witnessed success with three blades.
Since you have a Brian Eather 12.5, I would try that but re-pitch it to at least 5.5, maybe a bit more if you can.
Since most of my 4-S flying is done with 56 and smaller engines, I haven't used a SAITO 72 in over 15 years, so there may be more modern props available.
Master Airscrew makes a 13-6 that might suit your needs and I'm told that they can be re-pitched if necessary. Not very expensive either.
In any case, experiment a bit and find the right match for your engine/plane setup.
I hope this helps.
Bob Z.

Hi, again.
Sorry, but I cannot help you on the re-pitching process.
I've heard of many people doing it but I have never tried it myself.
Try posting in the regular engines section and I'm sure you'll get some expert advice.

Bob Z.

Claudio,
I've repitched many CF, APC and wood props using a heat gun. The first thing is to either get a prather pitch gauge or make a Windy wood one (basically you get a section of motor mount drill a 1/4 hole and put a pin/bolt in then at the end, which is at the 70% blade length point, attach a piece of plywood at the angle of the pitch you want). I use the prather, I pitch at the 70% point and clamp the gauge so I get an accurate pitch for all blades.

The process is straightforward: heat the hub area (keep the heat gun about 2" away) by rotating the prop from front to back, takes about 30 - 60 sec as you let the heat soak in (you should have on a thin glove as the prop gets hot) then holding the hub and pitch slider, grab the blade at the 70% point that  twist in the direction you want the pitch to go. This takes a touch that you will get a feel for once you try it, but it does require a fairly good twist. Hold for 20 ish sec then check the pitch, repeat as need to get the pitch you want. This works on wood props also, just need to heat a little longer to let it soak into the wood center, can change pitch up to 1 - 2".

Best,     DennisT