Looking at building one of these, would really like one with larger extended mags better than 13 rounds. I am looking at a barrel manufacture for 45 acp threaded barrels. Need a comp builder and the rest of the parts would be special order. Also thinking of building one in 10mm, but there is no 10mm parts YET for Sig. Perhaps someone can start a comp to do this.
Does anyone have any clues how to get this started.

I think the largest issue you're going to have is, as it seems you've already identified, finding parts available for the 45ACP platform or having them made in 10mm. If you choose to go the 10mm route, not only will you need a threaded barrel and .40 caliber compensator, you'll need magazines made - A not inexpensive proposition by any sense.

Reports are the 14-round 45ACP mags for the P227 will work in the 45ACP P320, though I haven't independently verified that.

For other parts - whether you choose to go with 45ACP or 10mm - it will likely result in a costly exercise of having components custom made with no guarantee that they will function properly when all said and done. The market supplies what is demanded of it, and unfortunately at present there's just not high enough demand for accessories for the large-frame P320 to entice manufacturers to support it...

ETA; So I suppose to answer your question, right now I would suggest contacting various manufacturers with part requests/suggestions and encouraging other owners of large-frame P320s to do the same. If you show the demand is there for parts support for the platform, the manufacturers will respond to it.

I'd suggest reaching out to Springer Precision to start - They already make extended base pads for the 45ACP magazines, as well as comps and other accessories for the 9mm P320, so they're likely to be most receptive to suggestions/requests for other support

My suggestion would be to start building out the pistol with parts that are available now and add to it as manufacturers continue to grow support for the platform. A Compact 45ACP P320 to start, add a WML of your choice, send the slide off to one of the many good companies currently offering milling for optics (Jargerwerks and Parker Mountain Machine would be good for this), plus some mag extensions and maybe a trigger kit would make for a very capable package on its own. It then also gives you the option to continue adding the parts you want as they become available