I've been eyeing a set of them up too, just thought I'd chuck a feeler out to see if there were any of these around first.
Do you mean that you bored the main bolts a little deeper, and then tapped that extra depth? How far did you go, and how necessary do you think it is for an NA motor? I think I'd probably have to get a machine shop to do that, our pillar drill is a bit too shonky for me to trust it not to lay into the existing thread.
I've a few questions about the rebuild process in general that I'm going to put together in a thread, if you wouldn't mind having a squint at it I'd value the advice of those who've done it before!
Cheers