how well do these 5 lug conversions work?

Haha, well, I've only heard bad things about them like wheels flying off the car while you're driving.

All you would need are the spacers with lugs, 4 "topless" lug nuts per wheel to attach them and 5 new lug nuts per wheel to attach the wheel.

Unless you're looking for a larger track width, I would stick with a slimmer spacer, though the fox wheel wells do leave space for a little bit of pushing out. Good luck, hope the other guys can offer you more info!

EDIT: You could try looking for some deals on ebay for cheap 5-lug kits:

front: http://cgi.ebay.ca/79-86-ford-musta...oryZ6763QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
rear: http://cgi.ebay.ca/79-93-Mustang-GT...ryZ33728QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Just a suggestion, hope it helps!

dont use those, do it right