1)- The factory balencer for a Cleveland is not unique only to the 351C, so any 351M or 400M balencer should also work. Just be sure it's a 4-bolt type so your crank pulley fits, and the rubber bonds have not broken, as I'd expect from a 30-yr-old used balencer. When (not IF) the bond breaks, the outer ring will migrate backward carrying your timing marks with it, making it difficult or impossible to time the engine. All the vendors carry rebuilt units, pretty reasonably priced.
2)- All the Pantera vendors also carry clutch masters and long-throw slaves, some in stainless steel. Call them up or find their web pages & compare prices- the ss long-throw slaves are sourced from an unknown modern car so that's identical to all the vendors. The clutch master is particularly difficult to remove & replace, due to the extremely confined spaces under the dash, and the bolt pattern is peculiar so not just any master cylinder will fit. Some vendors sell stainless masters as well as hard-anodized aluminum that also resist water corrosion well.
