All this talk about drive shafts and ISYs & ESYs, I thought I'd confirm something in Jr's OP:
"Drive shafts: While there are several different length drive shafts used though the years, the physical dimensions of the 231, 242, and 249 are very similar. The overall length of each case is within + or - 1/4" of each other, per slip yoke configuration. "
My current tc, a March 93 249, has the ISY configuration, using a single piece rear retainer assembly, which measures 7 3/4".
My replacement tc, a Jan 91 242, also has the ISY configuration, using a combination of rear bearing retainer and extension housing, which together measure 7 1/2".