You never really said what is wrong with old engine, if its just high mileage and maybe burning oil or something but still running good, you also might look into having a machine shop rebuild it. With the economy the way it is some of those guys are hurting for business and may set you up with a good deal for the work and you would end up with basically a new engine. Down side is it will take more time as you will need to pull it out, let them build it, then put it back in.

If you do decide to replace engine, try to find one as close to your model year as you can. Sometimes in these swap out, even when using the same engines, there will be small differences between years that can cause you problems when you go to hook everything back up. Usually sensor plugs is what causes me the most grief, rather they are mounted in just a little different place so that wires don't reach or plugs are different so that new sensors on engine don't fit harness or something like that. I have actually found it is usually easier to strip down the engine to just a short block and put all your old manifolds and sensors on it just to eliminate or minimize this problem.

Just something else to keep in mind