Change Fuel Input in GUI (look/amount) [HELP]

Hello :)

I would like to add a alloy smelter to my mod. It should use Lava as fuel. The fuel should be stackabale so I can put in a particular amount

of Lava buckets to increase the Temperatur/working time of the smelter. 

So the Questions are:

-How can I do, that the containers just fills up when adding Lava and give back the bucket to the player

-How can the Input Slot be twice as Long in y-achsis direction?

Thank you for your help, I'm very new to this :)