I have a lead on a cheap 3.9 engine with ancillaries I was thinking about grabbing as a backup to my 4.2. From what I gather the blocks and heads should be the same. The idea here is to have a machined block and fresh heads ready to go int he case that my 4.2 takes a crap on me. What are the differences between the two engines mechanically, just the crank or rods as well? The engine is out of a '91 SWB and has 190K on the clock. This is being considered for my '93 LWB.