M&H engine harness difference small block vs big block 69 barracuda?

Get your engine harness from Evans:

http://www.evanswiring.com/default.htm
Note that he doesn't list all the part # on his website, but it's the same price for the engine harness, big or small block.

As far as your question, you are probably correct. The ballast resistor and horn relay are in the same places on the car, and the actual difference in materials between the two is likely minimal.