Hard to say, depending on what distributor is in the engine. Assuming
aftermarket ?  Most are made for 440 & are used with a spacer  for the 400

It has been at least 25 years since I worked on those engines. It seems to
me that I would look at where the distributor was located or the way the
spark plugs went into the cyl head, I just cant remember. Do a google on
it and Im you all your questions can be answered.

Glenn Beasley
Chrysler Tech

I have a '75 duster with a non original big block in it which I was told by
the shop I bought it from was a 400 (they did not install it, they
accquired it with the engine in it). However, the guy at the local muffler
man who put my exhaust on says he's almost sure its a 440. I looked for the
engine ID number in both the locations listed in my Hanes repair manual for
these two engines, but did not find the numbers, I'm assuming due to the
crappy repaint the engine seems to have undergone at some point. The block
casting number is 4006530-400-3. I checked this out on google and found
that the casting number 4006530 was used on '74-'78 440s and '76 to '78
400s, but no info on the last 4 digits. Does anyone know any other way I
can tell what this engine is? Some way to measure the deck height maybe?
Is the "400" in the casting number indicative of displacement? It would
really be helpful to know which engine this is so I can order the correct
parts for the upcoming build I'm planning. Any advice would be
appreciated. Thanks!