I own a 700 classic in 350 Remington Magnum, really can't imagine liking one in a 600/660 unless the size reduction was an important factor. I like the 350, a good caliber. In an attempt to revive or re market this original short magnum, Remington changed the twist rate on the last run of these. I think Ruger followed or perhaps led that parade, in any event, 1:16 is a decent twist, ideal for shooting cast. I shoot cast for practice, saving the big bang and spendy bullets for where they are needed.
The faster twist rates are better suited to bullets heavier than 250 grains. IMO, going to a faster twist was not an improvement, and the shooting public didn't either. Remington discontinued production before the year was over , lack of sales.
The Remington 600/660 and 700 SA all have a 2.8" magazine, which is a huge performance issue, when seating bullets greater than 200 grains. The Ruger 77 has a longer magazine, seating 250 gr spitzers is routine.
Reloading data is somewhat rifle specific, most manuals exceed the 2.8" OAL, when their test rifle is the Ruger.
I'm not seeing too many new 35 caliber rifles being made anymore, 35 Remington, 358 Winchester, 35 Whelen, 350 Remington Magnum, and 350 Norma Magnum are all pretty much obsolete. The only ammunition I see is in 35 Remington and sometimes 35 Whelen.
My next project will be some more cast testing and I have some Woodleigh bullets I'd like to try. The Woodleigh will need to be seated long, I will load these single shot. I'd also like to try the 225 Nosler Accubond, another single shot loading. I also have a box of 225 Sierra bullets that will work seated short.
My go to bullet is the 200 gr Hornady FTX, because it is available. It is accurate, however I have not tested it on game. CFE 223 and Varget work well. I'm getting 2800 fps, with a load that is a whisker under max.
I keep bringing it to gunshows, hoping to cash in, but so far no takers. If I have to be stuck with a rifle, this is not a bad one to be stuck with. I have a stockpile of bullets, more than enough new brass, and Varget is not hard to find. If I could find a LEE Loader in 350 RM, it would be a perfect bug-out piece.