My humble recommendation is look for a complete upper assembly that has your rail top since there are so many upper assemblies available with nearly every configuration possible. I like the Rock River lowers since their two stage trigger allows accuracy that I would have declared impossible just 20 years ago, and their uppers snap into the lower receiver with a tight fit. I prefer the forward assist as I shoot 77grain moly coated bullets, but to each their own.

 

My varmint special is the Rock River Predator Pursuit which uses a heavy match stainless barrel chambered in the .223 Wylde Chamber (allows use of .223 commercial or 5.56 NATO without headspace issues). This upper assembly has a free floated handguard, and retails about $600 with the bolt carrier group and charging handle assembly. I use a Millet scope, and can take out coyotes at 600 yards with single shots with this set up.

 

Cheaper possibilities are use a DPMS upper without the bolt carrier group. If you use the bolt carrier from your other upper, it is critical to check the headspace. That is why I like that .223 Wylde Chamber so much. Good luck.

You could do any of these.

You could get a mount that puts a Picatinny rain on top of the handle and a stock riser, http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=643824&t=11082005, that gives a good cheek weld. It looks kinda silly, but it works well.

You can amass a bewildering variety of uppers and lowers and stocks and wonder what the heck were you thinking.  Don't ask me how I know this.

If you change the barrel on upper(s) check the headspace or have someone savy check it for you.