For something large and heavy, like this, I'd prefer to avoid shipping charges. Where does a guy go for decent quality jack stands? Auto parts store? Hardware store?
Looking at 6 ton models:
$62 from walmart,
$50 from harbor freight, $7 shipping to a random CONUS zip code.
$98 for NAPA, but might not be in stock. 7 ton for them, not 6.
Show me a jack stand that won't sink in the mud a lot faster than an 18" square solid plate, and I'll consider it.
Then toss the plate down on the mud and put the jack stand on top of it. Or even a piece of plywood, I've done that before.
Same concept as putting a puck down on hot pavement to keep your motorcycle's stand from punching a hole in it.
According to our engineer, if I wanted to put a weight rating on a stand made from 4x4 11ga tubing loaded straight down like that, it would be somewhere north of 25 tons, (handy, since we put some huge loads on much longer sticks of the same stuff, and those have a lot of lateral force when the wind blows too) so show me some stands in that range that don't cost more than a car.
Excepting that, as Nick stated, you might not just have to worry about vertical loading - any side loading and you're getting into an unsafe condition. It's not that the stock can't take it, it's the question of the welding.
Not everybody has access to professional welders like you do.