For working on round firewalls, you need to find yourself a "center finder". I got mine from Lowes or Home Depot, I think, or maybe a woodworking store. It has ridges set at 90 deg on one side and 60 deg on the other for smaller items. Just lay it across the face with the ridges tight against the item. mark a line, and then rotate about 90 deg or so and mark another line. Where they intersect is the center. I used the lid from a butter dish to illustrate, but it's the same on a firewall.
Also you can use this chart to figure out how much to offset and engine from center: http://www.lcrcc.net/offset_calc.htm