well that +3 for the astro seats and I think I have to agree!
Yeah the fabbing the shoulder belt doesn't look too hard. Here's a link to Deve's Tech Net where he goes over installing a shoulder belt.
http://devestechnet.com/Home/SeatBelts
He as some good stuff on his site, more along the restoration side of things but still great information.
Yup, I think I'll go with the astro seats. At least the tahoe's fold up and thus don't take up much space!