Shortfuzed80,

The guided plates failing at the base of the fork is due to binding of the pushrod/guide either laterally and/or at the inset of the fork. Under dynamic conditions the pushrod will rub against the guide fork creating the potential for a fatigue related stress fracture, a result of vibration as the pushrod exerts force at its base. This condition can be exacerbated by deflection of components. The rocker stud should never be torque into place prior to inserting the pushrods. While there is little movement, having the guide loose will allow any marginal adjustment against binding when the PR is inserted.
Alignment problems are not uncommon given that differing cylinder head designs have variances in stud placement, multiple compound angles, 24º and 26º architecture, cant and splay to further complicate matters. The best solution is to cut, realign, and weld back your current guides, or better yet, source the adjustable guide plates. Be sure to leave the adjustment loose while you align your PR, only tightening it when you have minimal interference on both pushrods. These plates also do away with the need to elongate the slot in the fork.

Bob

i just want to add,i have used darts adjustable guideplates many times with great results,im sure bob has them in stock.