1/2 inch cold rolled over a 3ft span would be way strong enough so your 5/8 rod is PLENTY. It takes very little pedal pressure on a boosted brake but even if you lost boost it would still be fine. We have used hanger bushings at the halfway point not to prevent deflection but to dampen flop on a rough road. On a 1930 Plymouth that had a very crowded underside with the exhaust, transmission cooler, and an a/c condenser with fan we had to mount the m/c just in front of the rear axle. The rod clevis on the pedal was moved to the opposite side of the pedal pivot pin so it pulled instead of pushed and then a bell crank was mounted to the frame just in front of the booster with just a short actuator rod from bell crank to booster. 3/8 rod, that PULLS, and is over half the length of the car has been working fine for 10 years. There are remote mount power brake boosters than can be mounted anywhere and we have put these in the trunk on very narrow cars with a cluttered area on the left side between the transmission and frame rail.
