Pete
I just happen to have my old plans handy so here are some measurements:
firewall to outside of cowl or to engine thrust washer = 5.25"
thrust off set to the right is 2 degrees as per plans
the center line measured down from the top of the fuse at the firewall is 2" (looking at the fuse from the side)
the center line measured in from the right (looking at the fuse from the top) is 3.25" and 2 7/8 " from the left. This obviously
allows for the right thrust.

I used a ST 90 on my GB Extra and it was a good choice. If I remember correctly Goldberg included some wood blocks in the kit
to space the engine out from the firewall to make it fit the cowl. They measured 5/8". Alot depends on the kind of spinner you are going to use also.
Also the CG shows at the rear of the main spar for the forward spot and 3/4" behind the main spar for the rear spot.

Hope this helps
Sam