Al, I have a couple of rifles with the scope mounted on the barrel block. That means the "base" is cantileverd back over the action. Since no base is that long, I use a long piece of aluminum stock, and mount a flat-bottom universal Davidson base to it. Available from Sinclairs . . .
If you don't have a flat-topped action, you could have someone mill the existing bases to take a flat bottom base. The Sinclair part number is DKB-12 -- or DKB-13 or BKB-14, for 1/2, 3/8 or 3/4-inch thick base. Personally, I use the 3/4 inch base. These are 7 inches long, BTW, so if you use the existing bases milled, you likely wouldn't need an intervening piece of aluminum like I do when mounting to a barrel block. Price for the base is (was) a little less than $25.00