Hi Fastbulbous -- Welcome to the Forum.
You were asking were to start:
1) There is a table and a schematic at the back-end of the Dynaco ST70 Manual (via the link from Roy) that shows representative voltages at various points along the circuit of a stock ST70.
2) I looked at your underside wiring picture but it is not very enlargeable, before it gets too fuzzy to read. So first can you upload a higher resolution picture, so that we can blow it up to see details of the wiring??
3) The things that your wiring picture called out to me are: (a) GZ34 Tube replaced by diodes, (b) there is some sort of jack on the front panel (to the left of the RCA Inputs) that replaced the Stereo/Mono Switch (possibly turning the amp into a mono only version????), (c) On the bottom right of the picture there are some additional capacitors, but I cant tell how they are wired into the circuit (don't know if they replace all or part or add additional capacitance to Quad Cap sections (Silver Tube) that is on the top of the chassis, (d) On the back panel someone replaced the two original speaker connectors with something better (two-wire as opposed to four-wire) but can't tell what speaker impedance is supported (4 ohm or 8 ohm or 16 ohm), you would need to check the colors of the transformer wires that attach to tell what ohmage is supported, (e) On back side, it looks like the ON-OFF Switch is missing (or replaced??). There may be additional mods, but that is all I can see now with the fuzzy picture.
4) If you don't already have one, I would suggest getting a 5 or 10 Amp Variac and use it at reduced voltage (instead of 120v output, use say 40v output, so that you would have 1/3 of the schematic or table listed voltage on the wiring side of the chassis when you check voltages).
Hope this helps, if you have questions, ask them.