What type of top do you plan? I got confused, too! I follow 7 1/2ft long. Is it a laminate top? Is it a stacked hardwood top? Why is the length a problem?
Formica over plywood - cut the Formica sheet to fit the wall perfectly, and a bit too big on the other dimensions. Get the back properly placed, glue down, then router or file the other edges to fit.
Stacked hardwood - plane to the same height. Carefully glue and screw each board in place on the counter, then sand the finished product.
Or am I missing something big?