Your battery negative needs to go straight to the block. An external ground can be supplied from the block to the alternator, but it normally grounds to the block through the bracket. Multiple ground straps from the engine/rocker arm cover/automatic trans and even the exhaust should bolt to the bulkhead / frame. This not only provide a proper ground but protects from galvanic action. I would suggest using "Star" washers found with internal or external "Tabs" to cut through the paint or grease. Always solder connections, use shrink wrap for clean look and extra protection. The factory battery ground cable provides a short connector for bolting to the fender and always provide a nice ground from the engine to the frame. Drill, clean with sandpaper, install "Star" washer on eyelet and place a dab of dielectric grease under and on top of the terminations for use in the "Salt Belt". Do a voltage drop with the engine running from each ground end you install, while the component has current passing through it with you DMM. High current readings will be around 0.50 volts (Like a Snow Plow) and small current circuits should test below 0..040 volts while on. Tutorials on voltage drop can be found on "YouTube". Just curious as I don't know the whole story but why are you using welding cable? Many "Amp" clamps are stand-alone and have decent readings for accuracy, but get one you can plug into your DMM to take readings? Lot of folks use 0/1 cabling for stereo amps, but it has to more with your distance from fused supply to device. Example; a 12 volt systems, a device 13-16 feet, that uses 35-50 Amps, requires a #6 cable. These charts can be found online. I am using my GM/ACDelco chart. But do not guess, measure cold turn-on and continuous, using a "Min-Max" feature found on quality DMM, that capture both current readings while using an amp clamp. Remember, this also serves to pick the proper fuse as it protects the wiring...not the component. Too big a fuse = fire and melted wiring harness. That's no fun.
