Cobalt327 is correct. The valve angle was changed significantly. Any camshaft with more than about .520" gross lift WILL touch the pistons. Pontiac Dude (Ken Keefer, Ken's Speed and Machine, Brooksville, FL) offers a great deal on "custom" pistons for the Pontiac, that can be had with the valve reliefs in the correct position, and still use a 389 bore. We're building a solid lifter animal for a '64 GTO right now, including the "long rod" modification. Should make a nice 450 HP, 6,500 RPM motor... Nothing like a TriPower goat at "full song"!
All Pontiacs use the same bolt pattern, as cobalt also said. Deck height is also the same, 287-455 (not including the 265/301 "short deck").
Do NOT use Flepro (or other "stock replacement") head gaskets unless you block the oil feed passage coming from the cam bearings to oil the rockers through the head. Later engines oil the rockers through the pushrods. A catestrophic loss of oil pressure WILL occur if you don't. If your 389 is from a GTO, it already has this done, and the provision to mount the starter to the block. '64 "big car" 389s still had the starter attached to the bellhousing. Those blocks can be modified. Pre-64 389s have no casting flange for the purpose.