You have to look at the size of the Pin...Colt is different from Armalite,Bushmaster.DPMS, etc...make sure you get the right size...now I have heard of bushings to make them work but have yet to see any!
Saw a Couple of very nice custom built Colt uppers at a gunshow in Atlanta today...would fit your application nicely...I believe the lowest priced one was $550 and the highest was $850...all were free floated heavy barrel nail drivers!
If you were looking for a social worker I have a preban upper I'd put on you for cheap...lol oh well that's not the case though...just keep your eyes open and check the shows sometimes you can find a good deal...just make sure to look at the pin size!