First of all, nice buy. You got these at a really good price.
Regarding the recommended reading from Curcio, Audio Regenesis, etc, please do that. It will help you understand the circuit and various options.
I agree on replacing the bias selenium rectifier with a diode, preferably a UF4007 or other "fast recovery" type diode. Also replace the bias caps with 100 vdc units.
The ST-70 was designed to run on 115-117 vac, unfortunately, today the AC voltage is usually 121-122 vac which leads us to a problem... I would measure the AC voltage on the output tubes, pins 2 and 7, these really should be 6.3 volts. Unfortunately they will probably be 6.8 vac or so which will shorten tube life and affect the sound. One solution would be to add a CL-80 to the hot side of the power cord. This is an NTC, a Negative Temperature Coefficient device that starts at a high resistance and as it warms up, drops to a low resistance. It serves several purposes: first, it will lower your incoming line AC voltage which in turn lowers the voltage on the tube heaters. It also reduced the AC in-rush current to the tube heaters, which increases the tube heater life by a factor of 4 according to Robert Tomer, a tube expert from the 1950s. It also allows you to use a regular fast blow fuse instead of a slow blo fuse which means if something does happen in your amp, the fuse will blow much more quickly. BTW, if the voltage on the tube pins 2/7 isn't lowered enough, add a 2nd CL-80 on the other side of the AC line.
One recommendation would be to remove the current bias measuring resistors and replace them with a 1 ohm, 1/2 or 1 percent resistor from pins 1/8 to ground. With that you can use the 200 ma scale on your meter to measure the milliamps going thru your tube while reducing the internal impedance of your amp. I believe the amp's internal impedance is covered in the Audio Regenisis article. I would run 40 ma per channel or so if I were running today's 6CA7s / EL34s instead of the 50 ma in the original Dynaco.
I would also recommend putting (at least) a 100 ohm 3-5 watt wire wound or metal film resistor between pin 4 of the output tubes and the green or green/white leads. This will somewhat lower the voltage on the screen grids but more importantly prevent an arcing of the screens. It will also tighten up the sound a bit. I say at least a 100 ohm, some people suggest up to 1200 ohm although I think that is high, I use a 560 ohm personally.
A third recommendation: replace the 6.8k and 22k resistors on the main power supply cap. The originals are carbon comp and they've probably drifted over time due to the constant load on them. I would replace them with Vishay CPF2 or CPF3 (2-3 watt) metal film resistors (was recently working on some other old gear, a 25k carbon comp resistor measured 18k).
The ST-70 is a very nice amp. I run mine with the Triode Electronics EF86 driver board and use an ECC-99 for the cathodyne phase splitter. I have the Triode power transformer and run KT-90s as the output tubes in triode mode. Of course, YMMV and you'll run yours as you wish. I'm sure you'll enjoy it.
Best of luck,