Yeah that's still a lot of juice - I really am very envious.
Microwave transformers. Look on instructables for loads of projects like that. I made myself a spot welder using a MOT, though other people have obtained better turns ratios I think I figured out the best way to wind the high current secondary, like this:
https://www.instructables.com/id/Spot-Welder-from-a-microwave-oven-transformer-and-/?ALLSTEPS
You see some where people are melting nails and stuff... Mine was too high a voltage at nearly 5v - should have been closer to 3v.
Oh if you're doing really high currents, you really do need to read up on ampacity - this is a start:
https://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity#Load_Carrying_Capacities_or_Ampacities.
I think it's a good exercise to rewind it to give you a voltage you can actually use for a linear supply - you'd get really good regulation because it's so big (someone please correct me if I've got that wrong!) Oh but wait you've already got a HUGE psu anyway. Hmmm
If you want to know how to work out how many turns you need, just wind a known number of turns (like 10) of ordinary wire onto the core, power it up and measure the voltage. Divide it by the number of turns and you will have the number of volts per turn. (Probably a little more than 1). Round it to 1 or 2 dp's to get a useful number. Don't forget the DC voltage you will ultimately get from rectifying and smoothing the output will be around 1.4 times the rms voltage (ie, the peak voltage), less the diode drops of the rectifier, which will be between 0.6 and 0.7 volts per diode (double this for a bridge rectifier), but then when a load is applied the voltage will drop due to a load of stuff I never learnt, but this is where the "regulation" of the transformer comes into play.
Don't fry yourself by accidentally touching the mains side. I was careless once, nearly killed myself, being alone in my damp basement flat...
Since you'd be able to draw quite a lot of current you might want to think about connecting rectifiers in parallel - if you do this you need low value resistor in series with each one. Same goes for pass transistors - you can connect them in parallel, just put a low value resistor in series with the emitter of each one.
You also have the opportunity to create a tapped winding with all the fancy voltages you could wish for. Make a centre tapped winding and you only need 2 rectifiers and only have 1 diode drop.
Read this superb tutorial:
https://www.electro-tech-online.com/threads/power-supply-design-for-beginners.113613/ explains it far better than I can.
If you know the exact mains voltage you can then work out the number of turns on the primary, so you would then be able know the exact turns ratio. I have no idea why you would want to know this however... I'm ranting now...
Good luck, have fun and be safe