For those who are offering their expertise but who have no clue what a Marlin MR-7 is, here's some photos of the rifle at the link which was produced for only a few years in the late 1990's. I also included some information in quotes from a thread on the Marlin forum about the design of the rifle. The rifle design included some of the better points from several of the well known production rifles at the time combined into one rifle. For anyone who bought one, they got a very well designed and functional hunting rifle for a very reasonable price. Marlin never marketed the rifle well, and it fell of the table rather quickly.
http://www.gunsinternational.com/guns-fo...un_id=100586094
"The Marlin MR-7 was only made from 1996-1999. It was only chambered in .25-06, .270, .280, and .30-06. It features a 3-position safety, a bolt face encircled by three rings of 4140 steel, a shrouded striker and high-visibility cocking indicator. It has an adjustable 3-6 lb trigger, A forged, machined and heat-treated receiver and an extra-strong bolt lockup. The Mr-7 has a hinged floorplate with a removable yet securely mounted 4-shot box magazine (The Mr-7B does not). It has a 22" barrel with 6-groove precision rifling and a recessed muzzle. It also has a classically configured, cut-checkered American walnut stock. The later (1999) MR-7B was made with birch wood for the stock and the MR-7B didn't use a floorplate. It was a few $ cheaper and in 1999 the MR-7B was only available in .270 and .30-06 calibers (I haven't came across any information for the MR-7B in 1996,1997, or 1998.)
The Ring of steel around the cartridge and the strong locking lug bolt-face was mimicked off of the Remington 700.
The wing-style safety was mimicked from the Winchester Model 70.
The "Dogleg" Bolt handle mimicked the Ruger 77
The floorplate with the detachable mag was mimicked from the Browning A-Bolt. (it is said that the Browning A-Bolt Magazine will work as a replacement, but I have yet to confirm this)."
In contrast, a simple M-98 action can be bought at most garage sales for $50 or a bit more in its original form. If you plan to make a functional rifle that will be shot a bunch from a M-98 action, you need to be aware of the year of manufacture so that you don't buy one of the brittle pieces of junk that Nazi Germany was trying to build while being bombed by the Allies 24 hours a day. They worked for German war production at the time, but some of them are metalurgical junk that I wouldn't trust to hold the pressure of a 22LR cartridge over time with considerable use.
If it were me, I'd hang onto the M-7 and have it re-barreled to 35 Whelen if that's what you want. And Shilen would not be my first choice of barrels either. To get a standard M-98 action up to the conditions of the M-7 action including a new trigger and a new safety let alone smoothing the action up, you're going to spend some big bucks. If you want a Mauser action, I'd look for an FN or a commercial action which can still be had for a very reasonable prices versus a standard military M-98 which is definitely nothing special.
Any decent custom rifle builder will tell you that if you bring him a military grade Mauser 98 action that you better be prepared to spend some serious money if you want a true custom rifle. If a bent bolt and a Timney $25 "scope safety" with the original military floorplate are your thing in a "custom rifle", you might get buy on the cheap if you get a functional and safe action to start with.
For the Op, I'd look closely to what has been done to the rifle in terms of improvements to the action and definitely look at the action date and decide from there if the trade is worthwhile. Even a good pre-war 98 action is rough if its not been improved a bunch by a quality gun smith.