[davepl]

I had a built 200-4R live behind a ZZ502 with drag radials, and it was fine, but it was a $4000 transmission, so not exactly stock. So they can be made to live (the 200 is weaker than the 700 stock, but built tends to live longer than the 700) but out of the box they won't support a big-block for long.

Is it possible it was already strengthened up by the previous owner, or did it just come out of a junkyard, etc?

If the cause of the slip is the TV cable, it's probably already too late, but worth checking the adjustment on it. If it's wrong, the trans will slip and eat itself. It's not like an old kickdown cable, because on the 700 it regulates internal pressures too.