Hopefully you didn't replace six 6V batteries with six 12V batteries.
If you did, you went from a 36V battery pack to a 72V battery pack and may have destroyed the controller, solenoid and motor, and are lucky it only smoked instead of catching on fire.
If you went from 6 x 6V to 3 x 12V you still have a 36V battery pack, but only about half the run-time. (Unless you used some 12V monsters that weigh 150 pounds, or more, each.)
Those old labels are hard to read, so you've got some numbers wrong.
Curtis 1205-1xx controllers are rated for 24-36V
The last 3 digits of the motor's E-Z-GO part number are "G04" rather than "604"
Also, the 3,000 pound capacity includes the weight of the vehicle and batteries, so only about half of that is left for passengers and cargo.
If you don't have a Service Manual, go to the bottom of the "1980-1999" column and download it. There are also parts manuals for it there.
http://www.bennettgolfcars.ca/ezgo_manuals.php
Personally, I'd start with making sure I had a 36V battery pack, then clean and inspect all the high current cables. (If the cables aren't at least 4Ga, I'd replace them.) Then start troubleshooting from there.
Hope this helps
John
----------------see You replied while I was typing.