If you've seen the pile of junk 4.6's I have you'd stick with the 3.9. If you want to go 4.6 start with a new manufactured block. The rest are hit and miss.
You "can" tune to some pretty impressive levels with GEMS, but really not worth it. Theoretically you should be able to get something special out of it and I have spent some time with this (110 HI-Cap with tweaked GEMS 4.6 and 5 speed manual), and would not do it again. That truck ate Salisbury drive flanges like they were candy, and went really well but again.....not worth the effort. I'd rather drive than turn my Defender into a Disco nightmare.
new expansion complete. Not only are we the only Rover shop in Eliot Maine...now we're also the biggest.
"Dedicated to the resurrection of junk through engineering?"