Autozone has a universal ground strap for cheap that will work fine from motor to frame.....
http://www.autozone.com/R,89628/stor...ductDetail.htm
For underdash, I didn't have a central metal mount for the gauges, switches, etc., so I went down to the local metal salvage yard and picked up a solid brass bar, 1/2" X 1" X 6". I drilled and tapped it for screws, mounted it under the dash and connected a 10 ga wire from it to the frame. Then it was a simple matter to tag up ground lugs from instruments and switches to the brass bar and much less wiring mess. Doing the same thing at the back of the car can clean up the whole thing. 10 ga wire with crimp on lugs will work fine for brass bar to frame. Lesser wire gauge, such as 12 ga will work fine from tail light/brake light to bar. Coat the lug connections with RTV to keep moisture out. Where you attach lugs to the frame, sand off a bright spot, attach the lug, then coat the whole connection with RTV.