I just bought a big block chevy engine out of a 66 biscanne highway patrol cruiser. Casting # 3855961 showes to be 65-66 396 and also 66 427. How can i tell the difference?

Engine code on the pass side deck. Hwy cars came with 427's, city cars came with 396's, at least in my home state of Iowa.

Colvin's book - Chevrolet by the Numbers, shows casting 3855961 only as a 396 used in passenger cars and chevelles. Passenger cars for a 396/325hp and chevelles for a 396/325hp or 360hp. A 427 for '66 is casting 3869942. Hope this helps

One other point is many of the early 396s can be bored to 4.25 or a 427 bore. The old rule is if a pencil won't fit between the center cylinders (accessed via the freeze plug) it can safely be bored to 4.25. You need to check for core shift to be safe.

I have heard stories of 961 blocks that were thick wall but I have not come across one. Like was said do the pencil trick it works every time.