Since you're going to be in the open class you might as well get the best, most aggressive gravel tire allowed. Off the top of my head I'd say the Michelin L/M forest series, or Colway Rally MS in a 195/60/15 (you can order these from [email protected]
England for about $50 each plus shipping) Another option is go to I think, www. pegasusautoracing.com and see what they recommend as to a gravel tire and throw them on a set of 15" steels. Or you can wait a few more days and ask the locals what they use.