If you're going to use that solder, double check the core. I'm not sure if this is still the case, but my understanding is plumbing typically uses acid core solder, while electrical wiring uses rosin core. It was a no-no to mix the two.
I'm surprised if they sold a pigtail that it wouldn't also include a lug on the other end for mounting to the frame, it's typically not a very long run from the negative terminal.
I think a desheath/twist or wire nut would eventually run into problems from the high vibration environment. I'd probably be most comfortable doing a mechanical butt splice since I'd worry about cooking the insulation or not getting things up to temperature. A crimp of that wire size (I'm assuming between 1 awg and 1/0) would probably require a hydraulic crimper though.
You could do a screw terminal butt splice, but I'd worry about vibration with that too.
Thermo-Shrink AL/CU Splice Kit is used to provide a reliable, yet simple method of connecting 600-Volt rated cables ranging from 4 AWG up to 1/0 AWG. Heavy wall, adhesive lined heat shrink tubing provides
www.homedepot.com
As for the heat shrink, you'd probably be ok using a lighter.