when you convert to propane you have to use a propane carb.
heres an auction for a whole setup less the tank.
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&rd=1&item=7976380857&category=33550
you need a propane carb(mixer), converter, and regulator.
i think the propane go to the regulator to bring the pressure down so that it can be fed into converter, and then the propane it fed into the carb where its mixed with air.
my dads toyota has a buick 225 in it that he has running on a 1 bbl propane carb. the hp goes down a little bit but the torque is about the same. now if you go with higher compression and you set your timing so it doesnt detonate, you can make tons of power with little spent on fuel.
neil