If you're talking about locating an Ampex tube 8-track from the '60s ... that would be super expensive and super, super rare and hard to find. Not to mention the size/weight and getting/keeping the thing running. I know someone who has one ... he's a fairly successful musician, and I'm pretty sure he can't even get his working properly. It would probably cost a lot of dough (even non-working), because 350/351 channels are coveted for their mic preamps.
That model would be called the 300-8.
Based on my understanding of the AG-300 8-track (solid-state, which I used to own), building an 8-track from something like a 300-4 transport with 8 350/351 preamps would be quite a challenge. These 8-track machines were essentially custom-built by Ampex Special Products. Ampex didn't offer 8-tracks as a standard machine until somewhere around 1968. You'd have to get two Sel-Sync panels (4 channels each) unless you were gonna record everything live and not overdub, 8-track heads, guides, etc. ... I can't remember all of the technical details, but there are issues like the impedance of the heads and the electronics being different, so you need a special transformer across each channel (which Ampex provided on the 8-tracks they put together), and all of the channels ganged together. I'm not sure the stock transport would even pull 1" tape (the 8-track version might be beefed up). Even serious techs might have a problem putting something like this together.
In my opinion, even finding and using a tube 3 or 4-track would be a tall order.
Basically, you're ultimately going to have to choose between having a mono or 2-track studio with a tube recorder, or a multi-track studio with a solid state deck.
