Thanks everyone, especially Bob. I've got my by-passes made and based on the good advice above, got my rough shape of the spike brought down on the lathe last night. The one thing not done, is the outer lead in ducting from the inlet lip to the start of the by-pass. Based on the info above, I'll bring it in sharper than I would have otherwise, letting it follow the back of the spike in, based on keeping a constant, approximately 20 square inches that it's seeing at the inlet face, and also let the back of the spike flow into the starter bullet on the engine. I've got the engines set about as far forward as I can, to help with balance issues, so I've got allot going on in a short space. The below pic, is a crude pic of what I'll look like. I can get close with this without a massive amount of effort... I don't have the bulge in the ducting around the starter bullet, but could add that later if it seems to have performance issues.

Thanks again all, much appreciated,
Lance