The 8 wire motors should be wired bipolar parallel.
Motors of the small Vextra size that WILL work well with the G540 and 48V are available from keling for $33 each.
http://kelinginc.net/
On this specs page is an example of Bipolar parallel wiring.
http://kelinginc.net/KL23H256-21-8B.pdf
Since you want such low speeds, you could also use a 382 or even 425 oz Keling motor.
CR.