I'm building a 16' version of Tom Yost's strongback, and the shop floor I'm building it on does not appear to be totally level. After bolting the 2x4's, together, I set the 1x6's on top and checked the level across every couple feet. Looks pretty dead-on, perhaps room for a very thin shim in a spot or two, but that's it.

On the other hand, running lengthwise I'm not so sure, and not sure how much it matters.

2 Q's

1. Can I screw in the 1x6's prior to attaching legs/stands to the 2x4 assembly, or should it wait until that's been added first?
2. Does lengthwise-level matter? I read past threads indicating varying amounts of importance.

Thanks! Hopefully my series of noob questions will provide a search-able how-to among other not-so-handy first time builders :)