Rather than hassle with replacing the gasket, I'd use a brush-on sealer like Copper Coat or Permatex Super 300 between the main cap and the old gasket. You're more likely to cause more leaks the further you tear things apart, and I doubt that the rear timing plate gasket can be found anywhere other than in a full overhaul gasket set.
Jerry
Yes, and very good advice. RTV has no place on any 1950's or 1960's, truck engines or what ever else.
