I talked to Paul about the RO-Jet .76. He said it liked bigger props/more load than he had on hand, and his airplane lost the killer corner he demands. I'm pretty certain he used it with a pipe. It seems to me that going to 4 blades (undercambered if possible) might fix both.
My suggestion is to email Brian Eather and make arrangements to buy some 4 blade props from him. They come "raw", so you can sand them and trim them as you wish. He also ran the RO-Jet .76 on a pipe for awhile, so you might ask about that.
Alternatively, Chris and Alan make CF 3 bladers for PA .75's that should work alright. They are made for Randy Smith, and don't know if they could be bought direct or not. I'd ask, if only because dealing with Customs delays and exchange rate would be a PITA.
Steve