Yes, I have the aforementioned fast ratio idler and pitman arms from Firm Feel, as well as the '65-'66 V8 center link. I'll be using stock 340 exhaust manifolds so I won't run into header clearance problems.
Did you have them from another project? Can you return them to Firm Feel?
You WILL run into headpipe clearance issues even with 340 manifolds. The factory OE headpipe is only 1/2-3/4" away from the pitman on 67-76 cars with upward pointed centerlink stud. Less with downward point stud pitman. And the factory headpipe is reduced diameter to just to give extra clearance. The TTI and Accurate 2 1/2" diameter 340 manifold head pipes have a tighter turn for clearance and they are very close.
For fast ratio with 340 manifolds, you need to get a custom exhaust shop to explore IF they can fab something for the headpipes. The person I know that did on a 1968 A-body with fast ratio arm and 340 manifolds milled his drivers side manifold at an angle for added clearance and still made custom head pipes.
Sort of a moot point on a 60-66 A-body since the fast ratio arm will not even bolt onto those cars. You need a custom centerlink. Either shortened 73-76 or full custom like Firm Feel sells but with pitman/idler stud pointed other way. AND the Fast Ratio idler is through bolt, not stud like a 60-67 A-body. So you need to address that with something wonky like a just putting a bolt on through there. Or fabbing a idler mount from scratch or modified from a 68-76 style.
In short, it would take a a lot of fabrication, suspension measurement skills, exhaust fab skills to pull off fast ratio arms on a 60-66.
