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.
Choice #2 is to cut and weld a combination of bits and pieces from your B/RB engine mounts and the lower parts of your slant 6 mounts to allow the BB to sit in the /6 saddles. There is trimming that needs to be done to the K frame in either case for filter/pump clearance, you'll have to test fit it a couple times until you feel comfortable with the install.
There's other ways, too- these are the easiest.
