Thanks for the replies. Apparently the person who is interested in the project didn't like the #1 conversion as he didn't want to look at exposed rim when he closed the block. I thought that was silly but it's his gun.
Available option 2 was a b-78 but that might be too small in the threads.

Any possibility of using a single 10ga as the receiver? I did the bolt thrust on it and it appears to be too large of a difference but I am willing to defer to someone who has done it.

I too like the other big single shot actions, but I don't think he is interested in spending on an action what he planned on spending on the project.

Tom