Originally Posted by d.b. cooper
The trans can stay in the same place----no need for a new crossmember.
FWIW, when I replaced the 250 six with a 350 in my 82 C10, I needed new "clam shells" (the brackets that mount to the engine). Also had to relocate the frame brackets to another set of pre-drilled holes on the crossmember.
Keep in mind there are several different models of clam shells, each with a different engine mounting height.