Fred, I would not use Deck-Blocs. In a few years, some will sink more than others and will leave some low spots or unsupported spots in the floor of your shed.
What you use for a foundation will be determined by how high above grade (or how close to grade) you want the floor of your shed to be.
Personally, I like the floor of the shed to be as close to the ground as possible.
If it were me, I would dig 3 trenches, parallel to each other, about 12" inches wide by 30' feet long and at least 12" deep. The first one being right on the centerline of your shed, and the other 2 being exactly 5' away on either side.
I would then fill those trenches with about 8" of compacted 0-3/4" gravel.
I would then lay some 6x6 or 8x8 creosote treated (cheap alternative Used Motor Oil) Hemlock beams in the trenches making sure to level them perfectly with a string line or a laser. The beams should be 10' long (very heavy) and can be joined together by nailing 8' 2x6 or 2x8 boards to either side. OR, you could build some forms out of 2x6 and fill with concrete.
I would then lay 2x6's perpendicular to the beams to construct my floor and use a 2x6 skirt board all the way around. You could use standard 96" 2x6's and overlap them along the centerline beam. Spacing of the 2x6's will depend on what you will be putting in the shed. If it will be heavily loaded, 12" on center. Otherwise 16" on center.
Some wire mesh stapled to the outside and buried 12" into the ground will prevent critters from making nests under your shed.
You should be able to get the beams for a pretty affordable price at Scierie Ste-Sophie on Hwy 158.
~Phil.
