A lot of guys have some type of mount attached to the frame and the battery attaches to the mount. Needs to be 3/8 hardware.

A couple of other things I might suggest are

A) Use Quality crimp terminals, a lot of the kits have cheap small lugs that don't have much surface area for a good connection. Industial wiring lugs are available with 2 bolt holes that can make a much better ground connection

B) Use good cable that is oil/high temp rated (some welding cable is not) type SGT (PVC) or better SGX (cross linked poly) Or spec cable. If it strips easy it nicks easy

C) A little high heat insulation sleeve can really save grief when routing cable by headers or hot sharp stuff