Bit late getting to this topic as Yeti has already made his decisiion and I was busy over the weekend. But I thought I would add some comments as Rob has pointed out I have put a 998BP Testastretta engine into my 748RS chassis. As some 999 engines aren't to different from the 998 I suspect I could have made one of thos fit as well. In my case fitting the Testastretta engine was time consuming as I tried to get hold of 998 bits as a lot of them are different. These include the Harness, ECU, Radiator and Fans, Oil cooler, Airbox and Exhausts. The Airbox is larger than the normal 916 airbox so ideally needs the 748R/RS/996SPS Foggy Rep type frame. The Testastretta engine also uses 10mm engine mounts just like the 748R/RS. As all the bits for the harness took such a long time to source I started to think about trying a 748R complete harness mated to the 998 shower injectors and then would have had to have got a custom Chip mapped to suit.
As for mikesps's question about the crankcase swing arm pivot I would be interested to know the answer. As a future project will include a 999 and my Dymag Carbon single sided swingarm.