High current, low headroom linear regulator.

[CJay]

How would you rectify and regulate the output of a toroidal 8-0-8 transformer that's capable of in excess of 70 amps to 13.8VDC and be able to supply, say, 60 amps to an attached load?

I've got two of the transformers (they weigh a *LOT*) and I could put them in series but that means I waste a ridiculous amount of power as heat as well as probably putting my back out...

[ovnr]

Rectification: I'd probably go for some LT4320 ideal diode controllers and some ultra-low-Rds(on) transistors. You could do 60A with less than 5W loss.

Regulation: A 60A linear reg is a bit exciting, but all you really need to do is scale up a smaller regulator circuit. You will need a rather hefty heatsink, however.

[Zero999]

How would you rectify and regulate the output of a toroidal 8-0-8 transformer that's capable of in excess of 70 amps to 13.8VDC and be able to supply, say, 60 amps to an attached load?

I've got two of the transformers (they weigh a *LOT*) and I could put them in series but that means I waste a ridiculous amount of power as heat as well as probably putting my back out...

This isn't possible without wasting crazy amounts of power. The peak output voltage of 8VAC is 11.3V, so using full bridge rectifier, with two diodes, wouldn't give a high enough voltage. The peak voltage of 16VAC is 22.6V, so you'd have to waste a lot of power dropping it down to 13.8V. A low drop-out regulator will not be required. A TL431 and an array of Darlington transistors on a gigantic heat sink aught to suffice.

I wouldn't do it. I'd get a switched mode power supply to do that.13.8V is a fairly common voltage for automotive applications and is widely available.

[mariush]

Using something cheap and easy to buy ... maybe 10+ LT1084/LM1084 linear regulators each with a resistor of around 0.2 ohm on the output (to balance the current)
Check the LT1083/LT1084 datasheet, they have an example showing two lt1083 (the 7.5a version of lt1084) in parallel, for a 15a power supply.
 
You'll need a bunch of bridge rectifiers as well, it wouldn't be a good idea to use a single one for 60 amps of current... not only because you'd have [ 2 x voltage drop x 60 A ] watts dissipated on a single rectifier, but using multiple will mean lower voltage drop on each diode of the rectifier. 
Maybe something like GBJ5006 or GBJ5010 (50a rectifiers easy to heatsink)

[MagicSmoker]

You could use phase-angle regulation on the primary (using a triac or back-to-back SCRs) and a full wave rectifier feeding an LC filter on the secondary. This is essentially a mains-frequency buck converter, so relatively high efficiency (especially since regulation is on the primary side) but terrible transient response (loop bandwidth will need to be less than 10-15Hz).

[Kleinstein]

8 V AC is to little to give you 13.x V DC. With both windings in series there is a problem getting enough current: with a simple rectifier and filter 70 A AC current would lead to about 35-40 A of DC current only. So to come close to 60 A, you would need a kind of PFC. Doing it passive, with an inductor is heavy (might be as much as the transformer) - the good thing is that PFC would reduce the voltage and need less filter caps.

Still getting a suitable switched mode supply is much easier. !3.8 V is common.