I looked at doing a jag style by modding a kit but opted for a (slightly) more traditional Jazz style... build diary below

http://www.buildyourownguitar.com.au...ead.php?t=4071

To go full jag style by modding, buying the correct neck, hardware ect would potentially be more than picking up a Squier Jag and modding/refinishing to your liking... I ended up grabbing one off ebay for under $500 with upgraded Mustang bridge and it is a killer guitar.