Large seat tube.

I am building mine on a similarly thick frame
Even the seat post needs a large frame adapter

the front tube needed one of Sick Bikes adapters, which does use an exhaust pipe clamp

It would b easy to use an exhaust clamp only by simply drilling holes to match the mounting studs and then using either the supplied U bolt , if it will fit, or a larger version of the clamp that comes in the kits
http://www.custommotoredbicycles.com/yahoo_site_admin/assets/images/IMG_1200.28784829_std.jpg

Because my frame is aluminum i wanted as much mounting surface as possible so I substituted one of those clamps and used two bolts to mount Sick's mount

This is what I am using for large frame mounts now

http://www.custommotoredbicycles.com/yahoo_site_admin/assets/images/IMG_1200.28784829_std.jpg

plus
http://www.custommotoredbicycles.com/yahoo_site_admin/assets/images/IMG_9791.11192033_std.jpg

I think it has a much cleaner look

I used that set up for the large seat post
It would work on the typical large front tube on most new cruisers

For the back I substituted that clamp for the exhaust pipe clamp u bolt in the Sick Bike mount

Sick Bike Parts

You could also just drill the clamp part to fit the engine mounts and use the clamp on the back side instead of buying the motor mount

I hope you uinderstand that, I'm not good at explaining these things

http://www.sickbikeparts.com/Manuals/SBP2 Manual001 Rev A.pdf's

This is part of the shift kit, take a look at how it mounts, won't need the jsckshaft stuff, but a shift kit IS the way to go. Contact Sick bike parts, maybe they will sell you just the bracket