What won't fit? I always put the bottom of the caliper in first then it takes a little persuastion to get the top in, then I push the top all the way forward, and take a large screw driver, or small prybar and lift the caliper all the way up and forward, then you can get the little slide and spring in.
Did you clean all the rust and dirt off the areas where the caliper slides side to side? Did you put caliper grease on the areas where it moves and the little slide you drive in and out? This will help greatly combat the caliper sticking causting brake steer. Whenever my truck sees lots of mud or deep water, the calipers come off for cleaning and lube.
Michael