Al;

6V systems indeed have heavier wiring to accomodate the increase currents, but voltage drops invariable occur not in the wiring, but at connections!

"group 1 size" and even "1" gauge means very little to me...what is wire diameter in inches or mm?

Poor connections at end terminals of bat cable (even hidden inside crimps or clamps, due to battery acid migrating down the cable strands) can cause a significant voltage drop under the extremely high currents which a 6V system draws (and in a 6V system, there's not a lot of voltage in the first place...loose a volt here and there and you're all done!).

The best way (short of a "four wire" resistance measurement of the wire and connections of your battery cable - and ground strap, but this requires special equipment), is to measure the voltage drop with a multimeter, under the high current conditions of starting...locate the voltage drop, and you've located the inadequite connection (inspect, clean, tighten, and protect for a long time with, naturally: Anti-corrosive Paste (ACP))...the poor connection will likely also develop some heat due to I2R heating...so any spot warming up or worse (smoke or sparks) will obviously be a prime suspect.

For those who insist on keeping an original 6V system (and there's nothing inherently wrong with it, it just more prone to problems if neglected), a preventitive cleaning, ACP treatment is highly recommended.

Cheers!