Like Bill said.

You can build them any way you like. Download the Estes instructions and fin layouts (you can get most if not all here). You might also download the 1974 Custom Parts Catalog from Jimz - it will tell you how long the body tube is based on its part number (BT-60R is 5" long). Or you can look up the same info on Ninfinger.

As for motor mounts, that is up to you. Personally, I think one thing that makes a Goony a Goony is the 13mm motor mount. Just substitute a 1.75" length of BT-5 and two RA-560 centering rings for those in the Baby Bertha.

Upscales and downscales are definitely possibilities. Upscale by building from a Fat Boy (or buy a length of BT-80 and a PNC-80BB). To downscale, use a BT-5 and PNC-5V (Mosquito nose cone - you can still get them in the PNC-5 Nose Cone Packs). I would personally use a micromaxx motor mount in a BT-5 downscale if you want to get them back.

Just remember that Goony is as Goony does!

Greg