The problem with the fuel stiffner is that when you have approximately 1/3 of a tank left or less, and you go nose down, the engine can become fuel starved.

The thought is that you don't want to have the clunk stuck at the front of the tank where it can get lodged using standard tubing.

However if your tubing is flexible enough (you WANT thinner tubing for this) this should not be a problem at all, and you CAN permit the clunk to travel forward and rearward freely, provided that it cannot wrap around the fuel inlets themselves.

A longer piece of inlet tubing can prevent the latter.

You should always take the tank out and make sure that there is absolutely NO way that the clunk can get stuck and try to make it get stuck as much as you can, to check.

Many of the commercial tanks, such as the Dubro tanks, come with the thinner tubing that permits forward movement of the clunk, while prohibiting it from wrapping around the inlets or getting stuck.