Yep they're out there but a word of caution: they do not make the power you think they would. I had my 4.6 rebuilt with stock parts (except for the arp hardware in the bottom and the flat top pistons) and with full exhaust, tune, jlt intake she puts down 323 hp and doesn't stop making power till like 6K. From what i understand a head/cam package, full exhaust, and tune, would produce similar (or better) results with a 2v without all the swap hassle.
Check it out as we work on upgrading the head, cam, and intake on a '99 GT's Two-Valve 4.6 engine. - 5.0 Mustang & Super Fords Magazine
www.motortrend.com
This MHS Prepped Head / Cam Combo comes completely assembled with your choice of camshafts and our camshaft degree service.
www.modularheadshop.com
its a lot less work, probably similar cost in the end, and you'll get something that pulls all the way to redline. i'm a broken record, i always suggest this to people considering a swap because working on 4v engines in the mustang bay SUCKS dude.