The easy way? Cut the motor mount "stands" off of the big block donor K frame. Cut the slant mounts off your K frame, leaving the steering box mounts untouched. Using the trans mount to locate your engine in the bay, weld in the big block stands. It'll take a little trimming and jostling around to get the engine situated properly, but just weld them in and reinforce as you see fit. In a nutshell, easy peasy.