6.5x55 is a standard chambering for an MRC 1999, I have one - it's the one marked "Sold out" on the Whittaker's list.
As to custom chamberings or custom builds I do not think MRC does that and you might find it difficult even if they did. To be candid, they make a good production rifle and as long as they make what you want you're good to go, but with one or two exceptions dealing with the people at MRC is a less than satisfactory experience.
Two years ago when SAS was doing the MRC 260 Rem group buy, the 6.5x55 was not yet a standard chambering for MRC. I ended up custom ordering a LH MRC chambered 6.5x55 from Whittaker's with all the same specs as the 260 group buy (Stainless, #2 barrel, fluted, 1:8" twist, green stock). Each change required $$$'s, but MRC did it.
Fast forward to late 2016 when MRC released their ALR rifles. I worked with Eric at Whittaker's to custom order a LH ALR. I requested several changes, the most important of which was a FWT barrel contour to reduce weight. MRC originally told Eric at Whittaker's that they would do it, and then backed off and declined. I am guessing that MRC may be something like Savage, if they can build what you want out of existing parts, they will do it. However, also supposing that all of their LH ALR stocks were inlet for a single standard barrel contour, and they did not want to put the cost and effort into doing a one-off LH FWT stock inlet...
If there is a specific MRC combination that you are interested in, I would recommend contacting Eric at Whittaker's to see what he can order from MRC. SAS and Eric are both good folks to work with.
