Sounds like a great set up. I am strictly a manual trans guy. I have been using the mechanical linkage with long branch manifolds forever. That is the beauty of them - they are designed to fit with factory parts. Since you already have the mechanical linkage, why not figure out what the problem is? The Z-bar support plate that bolts to the subframe can only go on one way. The ball stud in the block is also fixed. The Z-bar should fit between them easily with no binding. The upper push rod should fit around the power booster, the lower rod should fit past the bell housing. Post what part is not fitting for you and we can figure it out. Perhaps trying to fit the parts while the fenders are in place is the issue.

As far as strength of stock parts, I advise reinforcing the Z-bar arms. GM relied on some cheesy spot welds to hold the arms. A heavy duty clutch like a RAM Borg & Beck style can sometimes cause the welds to fail. I weld Big washers around the Z-bar shaft at the arms, or a short support arm, to make the bar stronger. The upper and lower rods are OK, but will wear in time. The Speed Direct kit is a huge improvement. It uses spherical rod ends that are far superior to the stock parts. Speed Direct kit for 1967-68-69 Firebird