Tim:
A great source for advice on correct guage and lamp placement is a website that a gent in Australia has put together that details the specs of both 289 and 427 cars, street and S/C. It is:
http://users.pipeline.com.au/bryanw/
You will find diagrams of all dashes with the correct placement of gauges. They are, however, for RHD cars, so just reverse them for your needs (I am assuming you are building a LHD car).
As for hole diameter, mike your gauge diameter and cut your holes for a fit as close as possible. The ORIGINAL indicator light bezels measure approx 7/16".
A tip: When you cement your covering (the original S/C's used Vynide, not leather), be sure to let the application cure for a couple of days before you trim the holes, as most of these materials - including leather - will shrink, leaving you with exposed aluminum possibly outside of the gauge bezels.
Hope this helps.
Bud
