Using one of the Athearn floor sections, cut off the steps/platform so it is flush with the end of the caboose body. Now determine where you want to have the splice joint for the floor pieces (offset from the body splice location is best) and make a square cut at that location.

Repeat using the other floor section; remove the steps/platform, then snap the first floor in place on the body and place the other floor section in place so you can determine the cut location for that piece.

Cement the floor sections together, and reinforce the joint with a thick piece of styrene for strength.

The correct steps/platforms to use for this model come from the Altas Extended Vision Caboose. It's part #190001 (ICC caboose underframe) and costs $2.50 each plus postage. Check with your dealer or call Atlas @ 1-800-872-2521 to place an order. Cut the steps/platform off the Atlas underframe, and cement them to the Athearn floor.

The Athearn underframe pieces can now be cut to the correct length and spliced back together. The truck mounting  holes are already in the correct location, so you can still use the underframe locators to build up the underframe. New cross braces will need to be made for the 2 center braces; locate these using the HO scale drawing and glue in place.

The rivet strips are made from styrene 1x3's spaced according to the template below. You can emboss rivets into each strip if you want to.