Hey fazeka, don't know anything about the AG-350 transport but it may have a control box so different it won't work without a lot of fiddling. If it's more closely related to the 440's then go for the 350. I don't think the heads on the AG 350 will work with the 351 channels, wrong impedance; but don't take my word for it you might be able to wire them backwards and get them to work. The 300 had a big casting under the plates called the 'banjo' it provides great stiffness but the 300 transport's big drawback is it wierd shuttle mode lever. It also has a tire driven capstan; I like it but not everybody does, they claim increased flutter. I have a 14" 300 that's been modified to 350 style controls w/an edit switch and 350 channels that got skunked big time by genius engineers at Capitol. The electronics are a HUGE mess! It has a certain potential as a mastering deck but would require an awful lot of work. Keep in mind these guys can't bias up past 406, certainly not 456 without some major fiddling; and they won't fully erase 456 either. The 3200 duplicators are basically the same transport as the 300, but they've got duplicator heads of course. All these machines are forced tape guidance transports, Studer and AEG went way into precision tape driving mechanism that would not have to rely upon forced guides, again more flutter with forced guidance. If digital's problems are about sampling rate and dither, analog problems are about flutter and wow. Good motors and staright as an arrow tape path, no wow. Resonances in the tape path cause flutter, so if the tensions are off on the machine, or if the guides or heads are dirty or if the reel idler is worn to the point of becoming another tape guide the tape itself will scrape along these mechanical obstacles and start to vibrate, like a violin string and bingo you've got flutter. Digital seems to have sent a precedent for super precision time base, tape is way less than precise in duplicating time. Personally I don't know why time is so important. Anyway, you're right, these machines produced lots of masters, but your question is sort of like asking "Didn't Mercedes win alot of races in 1951?" A short but interesting thread on the Ampex list this week revealed there are NO video producers who have even the slightest nostalgiac fondness of tube based video recorders, digital is where it's at and nobody's looking back. Until, as also mentioned, somebody needs a old 2" Quad tape transfered to digital video.
That said, I'd say yeah, turn it back into a full on 351, it'll certainly have more collector value that way, & it will make suprisingly pleasing recordings, but it will never record like a late model Studer 24 track or an ATR.