cam, when I was doing liquid fuel I used dubro tanks because they don't have seams. I don't remember if the flex tanks have seams, but the reason no seams are so important is it will start to tear right at the stopper and leak. just an FYI.
I used to resize tanks by spraying pam on a cookie sheet, heating in the oven then pushing the end I wanted to resize down on the cookie sheet and leaving it to get solid again. some times it took a couple of these attempts to get it to fit. the cookie sheet lost heat fast so there was not a lot of working time. but the most important part was having a spare tank to get the temp right. I would put shavings on the tray and slowly bring the oven temp up till it melted the shavings but did not burn them. fyi again.
Joe