400 into 65 valiant

I only know about '67-up, but engines in those are offset 1-1/2" to the passenger side as well. Are they centered in the early A's?

The "Footnote" link says 1 1/4" for a small block in an early-A.

The crank centerline for a SB early A is actually offset 5/8" to the passenger side - according to TTI anyway. The difference between pass and driver's side frame rail to crank centerline measurements are twice that, obviously (1-1/4"). Mine landed at 9/16" to the pass side (1-1/8" total).

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How about front to back? Is there any kind of reference I could use?

I did what most folks do and bolted up the transmission mount to locate the engine and transmission front-back. Though you could put it however far back/forward you wanted depending on how much extra work you want to do.