A single side plate beetle box (later the better) will take 150hp all day long it treated well. The strongest box produced for a aircooled VW engine was the six rib (091) kombi box, it will take 200hp. If its a five speed your after then there are the Porsche box's to consider, the 914 which is really only as strong as the kombi box, its first gear is a joke, The 915 which is the pick for most due to its dimensions and strength and of course the g50 which is the pick of the porsch boys for all the right reasons but at a price. My advice would be to go for a 091 and save up for some Alvins gears. The box you end up with will withstand anything a EJ can throw at it. As far as adapthers go look no further than Bill at Uni Cap, he's the only guy to talk to in Australia as far as I'm concerned. Hey also check out the picture I've attached, the information is from the Kennedy Engineering website but I have no idea of its url so I make the picture. Excellent info!! good luck and email me some pictures.
cheers Stephen