Years ago when I ran a drag raced a 18 spline four speed I re-fabricated all of the parts.
The trick is to use a factory Z bar as a template, but re-enforce it so that there is 0 flex. I also re-built the pivot ball because it also showed some movement. I also welded a gusset to the mount on the bell housing.
I also used heavier clutch rods until I had it correct. Worked very well when i was done.
I used a Ram, Borg & Beck/ Long style clutch assembly after much trial and error. Contact them for what is the current best system. I know that spring pressures are quite low today and they are very reliable.
If you are going to drag race the thing, do it right and you will be happy.
I did the same thing... I changed the ratio on
the Z-bar on my 69 Valiant plus beefed it all up..
this was years ago so the heavy pressure plates
were the way to go... I was running a 4200# plate
and a 4400# plate.. on a 10.5# flywheel.. now days
the dual disc and other things are the way to go with
the adjustable weights(higher revs added more weight)
I also welded another plate to the frame rail with those
big pressure plates.. the first one started tearing out
the frame rail