Phase Control of SCR's; Hi Current

I know this is a little off-topic, but the talent level of the group
is so tempting...

Can someone steer me to a circuit for triggering SCR's at a user-
selectable point in the AC cycle? I've seen circuit snippets in asst
cookbooks, but not being an ee I'd like to find something a little
more complete. The rectifiers will be working at 50v 250amps max in a
mig welding application; duty cyle at max output would be in the nbhd
of 20-30%. Ideally, I'd like to be able to chop out the middle of the
AC cycle rather than the initial segment--but I guess SCR's don't
play that way. Triacs do, right, but not at 250A?

The block diagrams I've seen for commercial migs using transformers
with electronic control (instead of multiple taps) use SCR's as
described above. But wouldn't it also be possible just to use
ordinary diodes on the secondary and control the primary side? It
sounds easier to switch 240VAC at 40A rather than 250A. The
transformer doesn't mind getting an AC waveform with a bite taken
out, does it?

One final question. MIGS generally run "constant voltage" volt-amp
profiles, meaning that the output voltage tends to change relatively
little even though amperage may change significantly. So far as I can
tell by looking at several units, this is accomplised with a big
honkin' iron core inductor in series with the output. The inductor
tends to be about 2/3 the size of the power transformer with roughly
the same number of turns as the transformer secondary. A healthy bank
of caps on the order of 100k uf, is also part of the equation. (The
inductor air gap, if any, I don't know about.) My question is this:
Can the volt-amp curve be tuned--its slope changed--by phase control,
possibly active phase control during operation? This would make the
design of inductor less critical. My intuition is that there're too
many df in the problem to conrol with single parameter (trigger
point.) But maybe not ... I guess a scheme like this would require
microprocessor control and amps / volts feedback. There are some
interesting Nuts and Volts articles back in Dec / Jan about
microcontrollers and SCRs.

Thanks in advance.

--Jack