I've only done a couple of sheets of 8x10 like this but the 1000ws mono light was good enough in a 3'x4' softbox. Not wide open on the lens but of course I don't remember what F/stop. With 400 speed film it would have been even easier.
A couple of issues. It's real dark focussing. The modeling and room lights just didn't give alot of light to focus with. I'm thinking of using a 1000 watt work light to raise the room light level for focussing.
It seems like a lot of power to hit somebody in the face with.
There is a big difference between what one needs to make a nice image with 8x10, and what one would want if LF portraits were a daily matter !
I like 4800 ws. The big thing with 8x10 portraiture today is that sitters are not used to sitting still, however willing, and a little more depth of field is a handy thing because so many people have that silly idea they should be in focus when they pay a guy to take their picture.
I love the 'open sky look' for portraiture and shoot with a couple big softboxes as close to the subject as I can get them. As David said, that eats up lot of light. I'd venture 1200 ws as the dead minimum.
250 w modeling lights are VERY nice.
A neat thing you can do with a single head and a big flash pack is turn a small room into a big, soft studio... the look is distinctly 'open sky', everybody looks good in that light, and you can 'shape' the light as needed by simply putting up dark cards to lessen the reflection.
You can find good used strobes pretty easily these days.
"One of the painful things about our time is that those who feel certainty are stupid,
and those with any imagination and understanding are filled with doubt and indecision"