Lots to talk about.
1 the slope a 3.5-1 is as steep as I would go. The dirt may not slip on a steeper slope but why take the chance.
2. Either piece of equipment will move the dirt, what is cheepest.
Get a price by the yard.
The liner people should tell you about the gas vents. Is the price of the liner with installation? A one acre liner will be heavy and should not be installed by you unless you have a lot of HELP.
It may cost more to cover the liner but that is the best way to go.
Keep us posted.
