Greenhouses, Sheds and Sun Rooms - Need design opinions - bolts

I'm building a shed in my backyard, 12 X 20. I bought the plans for it, but I had to modify the design of the foundation because of a steep incline in the ground. (It's a bout a 3 foot drop over 12 feet.) The only modification I made to the plans was instead of sitting the 2x8 floor frame ontop of cement piers, I decided to sink 6x6 posts into the ground ontop of concrete footings. (I realize there is some debate about sinking wood into the ground, but its already done and dry so I have to go with it). And have the floor frame bolted to the side of the posts, like a deck)

The concern I have is that the original plans called for the 20 foot sides of the floor frame to have 2-2x8's nailed to eachother (essentially making a 4x8).
My concern is for the length of bolts I should use to attach this 4x8 to the posts, (the posts are spaced 4 feet apart, so there are 6 of them across the 20 foot sides). The guy at the hardware store insists that bolts that go 1.5 inches into the post (4.5 inch long bolts) is adequate. I'm nervous because I know that eventually the sill plate for the front and back walls will be resting entirely on the 4x8's, can someone either recommend to me the best way to bolt this board to the posts, or confirm that the bolts the hardware guy wants to sell me are adequate?

You're talking about lag screws, I think. I agree 1-1/2" isn't enough.

Can you buy 1/2" galvanised threaded rod, and send it through with nut and washer on either end? This would be stronger and less expensive, though you'll need a long auger bit.