I also have a 1957 Oval that has a Subaru 2.5 325HP engine in it. The early Bugs are all alike and will require a narrower radiator then what could be used in the 1968 and later bugs. I use a custom built radiator in both my 66, & 57 bugs built to my specs. When you have your radiator made you can order it with the water inlets and outlets made to your specifications. Your need to measure the width and height to determine what size you think your need. Mine are built to be about 19” wide, and 14” high, made of aluminum and double row cooling tubes. Basically the radiators should easily cool up to 400HP with adequate air flow. I currently use one single cooling fan placed behind the radiator as a pulling fan. If need be another cooling fan could always be added in front of the radiator for additional cooling.
See my earlier post on this subject.http://bb.bbboy.net/vwengineconversions-viewthread?forum=20&thread=26
Or also checkout my Websitehttp://www.bugaru.com