The cramped environment where the shifter support lives is hostile to all flexible plastic materials, due to the 150+ F temperature from the left side exhaust close by, vibration, road debris thrown up and oil dripping from the rocker cover above. A heat shield might help. It appears that there are many shifter shaft support rod-ends available commercially- Google "shift shaft support"- I don't know the exact ID or OD diameter we need. There are also some 'steering shaft supports' that look identical. But none show sealing boots. A phone call to steering U-joint makers Borgeson, Flaming River etc might pay off. Not everything can be resolved by e-mail.
Another factor is the roughness of the stock DeTomaso shift shaft tube's OD. To allow any rubber seal to live on a moving tube, the stock tube might have to be removed and polished with a drill press & emery cloth in the area where it runs back & forth in the metal support. This might be why D. Quella's setup requires a certain diameter shaft- not just for the OD thru the support but also for surface smoothness. I would error on the OD toward looseness to keep your rig from seizing up when things get nice & hot.