Howdy Back C nut:
I can verify that the "C8DE 6015 B" is a 1968 casting with dual bolt hole patterns for both early and late trans bolt patterns.
"C" indicates the decade of the "60s. "8" tells the year, "D" says it was originally a Falcon part, and the "E" tells you it is an engine part. The "6015" identifies it as a bellhousing. The "B" at the end indicates an engineering change- in this case to the dual bolt pattern.
This code does appear on 170 engines also. You can verify the engine size by counting the freeze plugs under the exhaust manifold, on the passengers side. 3 = 170, 5 = 200.
Ironically, the 200 will be more than a match for a short stroke 200 in the cruising department. Also know that a 260 V8 may not be the best bent engine choice as many factory 289/302 parts do not interchange with it. If I were in your shoes, my first choice would be a 200 inline six for a cool cruz'n Comet.
Check out the number of freeze plugs and the casting code for the head and report back. Any more questions?
Adios, David