The swap to the later K in an early B seems to work fine for bigblocks. As some have documented here, the firewall pinchweld need flattened and hammer work to the transmission tunnel will have to be performed.
The Hemi is a different story. You might want to get a copy of Mopar Muscle from a year or two back where Goldberg goes to put his HEMI in a '65 B-body using a later K. It didn't fit!
*edit*
Ok, found it:
"That night several calls and e-mails went out to ask advice from builders and owners of '65 Hemi Mopars. The responses all centered on the questionable use of the '66 K-member. everyone said they never knew of anyone using a '66-and-later K-member. A call was placed to the source of the original "use a '66 K-member" information, and he said, "I knew it moved the engine back a little." According to Schumacher, the distance is actually 15/8 inches. Nice for a drag racer who wants the rearward weight bias, is willing to hack the firewall, and who will not be using the original column cable shifter, but totally impractical for a street-driven car that is being built to resemble a factory A-990.
As the body came down over... read full caption
As the body came down over the engine, Goldberg realized there was a lot of room between the engine and the radiator support, and very little room between the engine and the firewall. The engine "fit," but not in the right location for a street-driven car.Goldberg made the decision to take a step back and use the original '65 V-8 K-member with Schumacher's Hemi mounts. That meant the tti headers would either have to be replaced by custom headers or be modified to fit. Because of the engine position, the '65 K-member needed to be notched for oil pan clearance."More here:
http://www.moparmusclemagazine.com/techa...r_cylinder.html
(I know this moves the motor back some. Does anyone know, if it will cause a clearance problem between the Hemi valve cover and firewall ?)