An aluminum rad will probably the about the only type you will find that will fit in that size limitation. They seem to work well with most dirt track race cars, so I don't see why they wouldn't work on the street.

You also need to make sure you leave enough space for the hot air to exit from under the hood, if the hot air can not get out, air to cool can't get in. This seems to be an area many hot rod builders forget about. Gene