I'm with the "split the span" crowd, a treated 4X6 with the tall dimension vertical, even just blocked with patio blocks every couple of feet will do a bunch more than scabbing on the plywood. Does your crawl space get exposed to freezing? If so you might want to dig a trench, back fill with gravel to build your supports on top of.
They make a concrete "footing" block for under decks, that might fit the bill.
This way you can string line your timber and jack where needed blocking as you go. You can also go back underneath and add blocking later if settlement occurs.
I'm afraid the scabbing will create the opportunity for moisture in between the pieces to help rot the 2X6's over time.
All the form loading books, I've got a shelf full of them, are for "short term use", something long term like a floor joist is different.
Shortening the span is the answer.
My 2 Cents,
Mike
Last edited by morecowbell; 05/20/13.
