You can't cut the shaft length down. Well technically you could if working from the top (head) end and your wire splices were good but I wouldn't try it. A 54" shaft TM is intended for a multi-species type boat like a Lund style hull. My theory on trolling motors is that you can't have enough thrust poundage. For what you are looking at my only question would be whether a 12V single battery system is sufficient. That is a good price however.
Take a look here for potential alternatives -
http://www.bbcboards.net/trolling-motors/