I have a Remington 700 ADL long action in 300 Winchester magnum that I am going to use as a donor action for a custom. I am thinking either 7mm Remingtom magnum or 7mm STW. My question is this: If I go the STW route will the action and magazine be long enough for the STW with 160 gr bullets so I don't have to seat the bullet down past shoulder and still fit the magazine?

I believe the Remington action will handle the STW length cartridge nicely.

a friend used a 700 in 7 mm mag to rechamber into 7 mmSTW.

The 700 long action will work fine.

The action is long enough, but what is the max length of the magazine? I looked at this conversion in a Savage rifle and the magazine length was too short to take advantage of the extra cases capacity (bullets had to be seated too deep).

To my knowledge, all of the Rem 700 actions made for conventional magnum cartridges have a magazine of about 3.6", which is adequate for "full length" magnums like .300 H&H and 7mm STW. The same action made for .270-.30-06 has an identical receiver length, but the magazine is shorter.

The inside dimension of the 700 magazine box is 3.680".I reload my 7mmstw's to 3.660"