Can 2 x 6's, stacked flat be substituted for the 6 x 8's? Would they provide the same structural strength?
You can create what's called a laminated beam from multiple boards (think plywood, but with boards). The key on laminated beams is you want those boards to work together. If the boards ever come apart, their structural support diminishes significantly, to the point that you could be looking at structural failure. As such, you want the boards to be both glued (use an exterior adhesive, since this is for boards that could get wet) and bolted (galvanized!) so they never come apart.
The catch here is that you're running the risk that some of those boards might not carry the loads you're putting on them. A 2x6 is more prone to develop some sort of serious crack than a 6x8. That is why the 6x8 is more valuable. This isn't to say it's an enormous risk (proper lamination helps reduce that risk), but it's a risk that is far more unlikely to occur in an uncut board.
There's also a math caveat. Finished boards lose half an inch, so a finished 6x8 will be 7.5" on the long side. You might be tempted to do this
8 / 2 = 4
That's wrong! A finished 2x6 is 1.5", not 2. So 4 boards gets you to
1.5 * 4 = 6
You need 7.5", not 6", so that's 5 boards, not 4. Be sure you factor that in if you go the laminated route. If the cost difference is not as great, go with the solid beam instead.