We need to know exact years, makes, models, and displacements you are swapping. It's not that hard, generally. Just pull one out and put the new one in! It's easiest if you swap transmissions at the same time, though. If it's a 4-banger, the rear end should be changed, but isn't totally necessary, same thing for the suspension. V6's are similar.
If you have a car with an inline 6 from the factory, you're going to have a real job to do. Trust me, I'm just finishing up with this myself. You must change the entire engine cradle/K-member/#2 crossmember in order for any other engine to fit. The inline six cars are the only ones you have to watch out on this for. Their K-members are VERY unique. Get back and post here if you need specific information.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1985 Mustang LX: a Kick-A white 4-banger
My website I can't even edit!