See my thread here:
A few posts down you will see how I used my 6Ah batts. They work well.
The Flexvolt batteries are very thick so that might be an issue. I don’t have any Flexvolt tools. How do you select the voltage?
Also, please post if you find spade connectors that fit in the plastic top piece and still are long enough to get a solid fit. I had to use make my own using .22 Mag shell casings. Also, consider using these at 40V. 60V’s would give you a lot of top end but at a slower cruise speed you will be running what I consider to be a very inefficient low RPM.