There are several forum topics in the 750 STOL area about seat belts and 4 or 5 point harnesses.  So far I have seen three different ways of upgrading to the 4 or 5 point harness from the stock over the shoulder seat belts.  Joe Harrington has posted pictures of his installation using a cable attached behind the baggage back wall and John Austin has posted pictures of his installation using a welded tube running from the existing shoulder belt tabs.  Both of these types of installation are done in order to get the correct angle of attachment.  The recommended angle of attachment is -10 to +30 degrees relative the the top of the shoulder.  If you want to do the conversion correctly, then you need to follow the FAA recommended installation guidelines.

Here is a post on John's installation.

 http://www.zenith.aero/group/stolch750/forum/topics/retrofit-4-poin...

Here is a post with pictures of Joe's installation and the other method I used.

http://www.zenith.aero/forum/topics/alternate-shoulder-harnes-att-b....

Now for the rest of us that don't always do things exactly right, but use the "that should be good enough" school of engineering, there is a third easier method.  Attach some brackets to the cross member just behind your head.

The angle of attachment relative to the top of your shoulder is more like 45 degrees rather than the recommended 30 degrees.  (Note: nuts were not yet tighten in this photo.)

And as usual, I also cheaped out on the belts themselves.  You can get some really nice sets of harness from companies like Crow that are made in America and come with installation hardware, customer support and instructions for a couple of hundred dollars.  Or you can go on eBay and buy some no-name probably Chinese made harnesses with no customer support, installation hardware or instructions for about 50 bucks.  That is what I did. DISCLAIMER: I am only telling you what I did,  I am not recommending this installation method or this type harness! Use at your own risk!

Harness did not come with the quick adjust tabs, I made then from an old dog/horse lead I had that just happened to be the right color.  Could not sew through them so they were installed with my favorite adhesive Shoe Goo.  ZA logo was printed out, laminated and glued onto the center of the cam lock release.  I made the pads from left over pleating foam and upholstery material.