Start off with figuring out your budget and intent then work from there. Write down all your thoughts and figure out how much it costs. Example: if I use a 350 ($), I need/want this transmission ($) that will work with it, I need this axle due to width, driveline, brakes. Can your frame handle the torque & HP of the engine?
As you take each thing apart, take pictures and when you think you are good, take more pictures. Put bolts in labeled bags, etc.
Just a few quick thoughts to get you going. This is not a quick project! I got in over my head a few times and guys in here helped me out. Understand, everyone here has an opinion, don't get offended when they are brutally honest with you. Do your research!
Good luck with your builds! Ask questions and learn before you do something!
Chris