The 400 will be the same as Mopar 383.
My 383 with Crower hydraulic flat tappet cam, Crane Gold rocker arms either 1.5 or 1.6 ratio, Mopar 906 iron heads, block decked .026 inch, heads cut .010 inch....
ball/cup pushrod length is 8.10 inch with 2 threads showing on the rocker arm adjuster as per Crane spec. Ordered the pushrods from Smith Brothers with oil thru to lube the rocker adjuster ball even though the rocker arms have adjuster oiling provision built in.
The OEM pushrods measured about 8.3 inch length, and worked fine with the above combo using OEM stamped steel rocker arms.
The Crane Gold rocker arms fit correctly on replacement rocker shafts from Sealed Power.
Hope this helps.