Lots of fun you've had using the ole' G503 as it was intended!
Can't get enough of Utah!
The Plymouth meet was great last April, too.
12V electric brakes will probably work fine on 6V. I had to de-tune the electric brakes on my Teardrop using an old fashioned adjustable resistor block, they were too powerful to control with the normal controller at 12V even at it's lowest setting. The resistor drops the volts seen at the brakes to around 3V. I'm running 3500 lb 10" electric brakes so I can have the HD axle and the 5 on 5.5 lug pattern. The trailer weighs about 1600 lbs, with 33" tires. 7" brakes are the normal size for that load, but you can't get the hubs to match the wheels without spacers and they only fit the light duty axle.
I used this one:
On 6 Volts, you might need it or maybe not. The electric brakes could be ran through this resistor alone, like they used to do back in the 60's. I think I paid about $25 for it on epay, in a Napa branded package, New, old stock.
e-pay search results:
https://www.ebay.com/sch/i.html?_from=R ... itleDesc=0