I just finished the exact same project on my bronco. However I performed all the labor myslef which save a lot.
I got a family cage similar to the Tom's version (
http://www.tomsbroncoparts.com/prod...ge-family-4-point-66-77-early-ford-bronco-new). Because he is local I was able to pick it up for $425 fully welded.
For the seat belts I ordered the Auto-lock brand belts (
http://autoplicity.com/products/95369-autoloc_11265_tan_3_point_retractable_seat_belt_1_belt.aspx). Seatbelts are not Bronco specific so I scoured the Internet for the best price I could find.
I needed headrests so I picked up a pair of 69 mustang low back seats off of craigslist for $50 and pulled the head rests off. The stock bronco seats already have the holes to mount them onto the frames. I needed them recovered to match my existing seats, but mom and her sewing machine helped with that (I may be 40, but I'm not ashamed to still rely on mom now and then).
For the rear seat I thought about adding head rests to the stock seat but ended up ordering the fold up seat from Tom's that already had head rests (
http://www.tomsbroncoparts.com/prod...ld-tumble-spice-40-wide-66-77-ford-bronco-new).
All in all, even with the miscellaneous costs for hardware and such I was still able to do it for under $750. If your considering an old Bronco, or any older vehicle, you may want to consider working on it yourself. Paying someone else to fix all the little things that go wrong can get expensive quick. And this is actually a really easy (good starter) project that really only requires some basic hand tools, a drill, and some research to figure it out.