There's two different clutch master cylinders used on these trucks, the early one with the metal body and diagonal studs was used 88-91, then the plastic one with horizontal studs was used for the rest of the run. The earlier automatic trans trucks did not come with the upper bracket welded to the cowl, whereas the later ones all had it regardless of the transmission. I think this change happened when the switched dashes but I'm not positive. Best choice would be to get a bracket from another truck and tack it in, next best would be to make a bracket. I think part of the cause of the issue may be that you're resting the top of the pedal mount bracket against the underside of the cowl; there should be a gap there and if your setup doesn't have everything sitting nicely it's going to flex and stress.