cobrahistorian, what kind of printer do you have? inkjet? or laser? cuz i got a testors decal system from walmart for $6 cdn and althought they say it only works with inkjet, I tried it with laser anyways and it worked perfectly. even the spray-can sealer that came with the kit worked perfectly. But the decal didn't seem to lay down flat easily. microsol worked fine with it. I haven't trie doing this with inkjet, since i don't have one. since the decals i made were all just black lettering, a black and white laserjet served perfectly.
here's a link to an article on ARC explaining the custom letters & numbers process; i basically followed this article but i only used what came with the Testors kit.
http://s96920072.onlinehome.us/tnt1/001-100/TNT045_custom-decals_Tucker/tnt045.htm
btw make sure you use the clear film when you print just black lettering cuz that'll make life way easier when cutting them out.