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I got a modified ST70 from Blair. It has replacement output transformers that I measured out to read 8K with a single 8 ohm output. The power transformer is 150 volt at about 600 - 700ma, good for a doubler, 6.3 volts at somewhere between 5 and 8 amps and a bias voltage.
What tubes to use with it? I was originally thinking 1625s since I have about 30 of them and it would make matching easier, but I would have to enlarge the socket holes for the pins to fit, plus drill more holes for the socket screws. Then I though of rebasing them to octals for a quasi 12BG6, but that would be a lot of work. It's difficult removing the glass from a socket on one tube, and I have to do 4 1625s and 4 bad octals. Plus I would rather not use anode caps.
What tubes to use with a 25 watt or so 8K output transformer of a ST70?

From your post I am assuming it has 9 pin sockets for the output tubes?
YES -
Then there are 2 obvious choices from the latter end of the tube era. Both were used extensively in US Jukeboxes.
I would look first at 6973 (ideal for the trannies and modern production available), the 7868 would also work but may in fact deliver more power than the 25W rated output trannies are designed for (depending on B+).
http://tdsl.duncanamps.com/show.php?des=6973
http://tdsl.duncanamps.com/show.php?des=7868

NO -
Not a 9 pin socket - then ignore this.

Edit:
Actually re-reading your post I see that it has octal sockets.
Class A push Pull KT66 would want to see 8K but the fat bottles may not fit an ST70 chassis.
Both the EL34 and KT77 will work with 8K at lower power and distortion than the more normal 6K or lower.
Another option is 7591.

Cheers,
Ian

Since the 1625's sockets didn't fit, I think I'm gonna try 807s. The sockets are just a little big, grinding them down will hopefully make them fit. And I'll try to squeeze in 2 X 6AQ7s and 2 X 6SN7s onto the driver board.

Will the power transformer fit the chassis and open some more options to different output tubes options

I haven't tried to fit it yet. I ordered 8 Russian 807s and I just got the sockets in. That transformer will for sure power just about any set of tubes, KT88s would work with it, but the output transformers look to be about 20-25 watts.

Ever build anything into this

I ordered 8 G-807s from Russia it took 2 months for them to get here. I got some phenolic 5 pin sockets and have managed to file one down to fit the hole, I don't want to drill anymore onto the chassis. I'm in the process, with Geek and Gingertube's help, of finishing my big amp, a 31 tube PPP 6GB5. It has taken a great deal of time to make.

I'm getting close to finishing Blair's ST-70. I have the Russian 807s, and in looking at the datasheet a push pull 807 fixed bias, needs a 100K bias resistor, which means I need about a 0.47 coupling cap. I have 8 - 0.47 500V Russian K42Y caps, but I would like to save them, just in case. Coop42 sent me 6 very nice Panasonic ECW-F(L) 1uf 400V caps. These things are big. I have a pair of Panasonic ECW-FD 1uf at 400 volts that are 7/8" X 3/8" X 1/4". Coop's are 1" X 3/4" X 1/2". I can't find a whole lotta difference between the 2 except the Dissipation Factor for the ECW-F is 0% and Pulse Handling Capability (dv/dt) Current (A0-P) for the ECW-FD is 11.4 I think amps. The ECW-F current (0-P) (A) is 72.4. I don't know how this would affect the signal going into the 807s, or if it will. Should I just use Coop's 1u caps?

You can see the datasheets on mouser's just click the link and click the first data sheet on each.

http://www.mouser.com/Panasonic/Passive ... c0&FS=True

http://www.mouser.com/Panasonic/Passive ... c0&FS=True