Fifth wheels work in the reverse of a travel trailer. They have a landing gear in place of your front stabilizer jacks that holds the weight of the trailer. The tripod jack is the fifth wheel's stabilizer jack and, just like a travel trailer's front stabilizer jacks, is NOT designed to hold the weight of the trailer.
With five 2 x 8's, That's about 7-1/2 inches. About the only thing I could think of that might help your situation would be a Fastway Flip jack( http://www.fastwaytrailer.com/flip/
). That would give you up to another six inches on your tongue jack. If your five 2 x 8's allow you to reach the desired height, you'd still need at least one 2 x 8 on the bottom.
I don't have a Fastway Flip (they weren't around when I bought my TT), but I do have an extended foot pad (http://norcoind.com/bal/products/con...essories.shtml
) that gives me about another six inches. The extended foot pad slides over the end of the tongue jack about 2 inches. I'm sure it's cheaper than the Fastway Flip but, but not as convenient. You have to slide it on/off and have a place to store it when you're towing- not a big deal in my opinion.
If you decide to buy either of these, ensure it's weight capacity isn't less than your trailer's tongue capacity.