Another option would be to add an auxillary motor mount to the back of the 170. Here's a link to several mounts from West Marine:
http://ecatalog.westmarine.com/0298....ckProdId=15006
Personnally I just use my big engine when trolling. It idles down so I can troll slow enough for Walleyes and White Perch or faster for Stripers. Also, you can always troll backwards to slow down even more if your motor/prop pushes too fast going forward. Just some thoughts. Let us know what you decide and please post some pictures when you get it all together.