Easy test if you can access the starter, run a jumper cable from battery too starter stud. If it kicks over easier, you know what the issue is! Use anything that can upsize the size of the circuit, even 2 12ga runs of wire would help.

Upgrade both side of the circuit, ground cable is just as important as the positive side.

Most of those trunk kits use a 2ga wire. Not sure about them if they are really old. In the 80's I never used that stuff, always 1/0 or 2/0 welding cables. My stuff starts even when smoking hot while hot lapping.