Old stiff wire casing isn't a problem unless its moved enough to crack it. Where is that most often seen? Field wires on alternators. I don't know why you want to increase the wire size unless you plan increased loads. Adding relays is the smart way to move loads off of OEM wiring and switches. Bring another feed wire with fusible link from starter relay into the cabin. Black wire to fuse box will signal a relay. New feed will power the hot in run portion of the fuse box. OEM loads of those fused circuits no longer on the OEM wiring and switch. If you want to feed all of that weld splice you will need 2 or 3 of 30 amp relays added under the dash. You could do the same with the blue with white tracer out in the engine bay. All this is just suggestions of alternative method, how modern vehicles are wired. Only relay I've added under the dash of my 67 is for the amplifier in the trunk. Its feed comes from where I joined the amp gauge wires during mad bypass and amp gauge to volt gauge conversion.