Blasts From The Past!
Back in the 1980's John Ciener would take your Colt rifle & modify it to use belted ammo. As part of the setup modified links had to be used in these conversions. Less than 50 were said to have been made (No longer available) & at the time it just cost a bit more than $1000. At today's prices a real deal, one of his units now sells for around $10,000 if you can find one. His quality is legendary.
What Else Is Out There?
There is the upper from BRP made from surplus MG42 parts. It's called the "XMG" it will attach to a AR-15/M-16 series rifle & allow belt fed firing.
Heres a link to their website. It may work great but if you want to keep the "Outline" of your AR you are out of luck, it still looks like a MG42.
The above was quoted from www.montysminiguns.com/beltfed.htm
Try a Google search on belt fed AR-15, there are lots of ways to go.
Regards,
GtG