well put beenthere, we all have a different view of whats safe, personaly i would do that provided the timbers were stout and the vehicle wheels were blocked so it couldnt possibly move, only thing i would do different is to do it on dirt rather than asphalt, on asphalt those timbers could easily kick out but on dirt you would be able to dig them in a bit so they wouldnt be able to slip out. heck i have dont things more or less by the books and had things go wrong, like when i had my baler on jackstands to remove the pickup head, it has to be on stands cause to get the shaft out both tires must come off. well it went great i crawled around under there for hours wrenching rusty bolts and beating and prying stuff since it apeared to be securely up on the jackstands, well the next afternoon i came home with the new bearings i needed and found it had fallen off the jackstands! thank god i wasnt under there!