Here's a link to some resin conversions done by Steve Faxon I believe these were done originally for the old Bandai line I don't know how well they will fit onto the Tamiya 1/48 line The page has his e-mail addy on how to get ahold of him
I don't know how good the conversion kits are I will be ordering one soon myself Probably the StuG III Ausf F

http://www.track48.com/model-pics/faxon/index-faxon.html