There were two different spring assists made for Cubs, one is IH and the other was made by Brinly, if I'm not mistaken. Either should fit your tractor, however the Brinly made assist clamps to the lift bar on the 3 point, on the right side in the tunnel and the IH made assist clamps to the implement lift rockshaft, in the left side of the tunnel.
Here's the IH type.
http://www.onlycubcadets.net/forum/s...=Spring+assist
I can't seem to find any info on installing the Brinley spring.