I had the same issue when I replaced my cast iron manifold with an Edelbrock RPM AirGap one. The adjustment was very easy (cos even I managed it

). There is a propper guide to this on a thread I started when I asked if the spring set up was correct but I'm not sure how to put the link up

In the mean time, see if my version and photos help, just note that these are photos before I set the springs up correctly so dont put the springs where these are
First I dissconected the rods at point A and C. Then swing the lever bracket up or down so you can insert a snug fitting drill bit through the holes at point B, there are two in the moving part and then through to the fixed one on the bracket behind.
Then my glamorous assistant got under the car to push the kick down lever forward on the side of the transmission box, this raised rod C but I still had to unscrew it on the adjustment to reach the new location, the factory piont needs a little down pressure to on rod C to be correct apparently but I had played about with mine to find the change point I like. Then clip rod C back on.
Then I had to adjust rod D so that it would line up with pivot point A and sit snug at the back of the slot (point E) without putting any pressure on the throttle lever of the carb. Since learning this I have made a few adjusments using rod D to find the best gear change point (mainly round the pits at Santa Pod when it was quiet

)
Hope this helps, I learnt this from the guys on here so someone may be able to put a link up to the pages I used from the workshop manual, which I promptly went out and bought
