Best approach/practice to design PCB which can handle 50-80A current?
I'm sure massive traces are needed to do this and which is not
  possible as I'm using Mentor Graphics PADS 9.5 in which the max trace
  width I can get is 0.25mil. Also heard of copper pours and planes but
  not exactly sure how to approach.


Sounds like you need to learn how to use PADs first but before that you need to understand what a copper pour is (do some googling because it's a general thing for all PCB software I believe). 

Anyway, go to setup and somewhere in that menu is a sub menu and try and find a sub/sub menu called "clearance" (it may be under a sub menu called "default"). On this page you can set minimum, default and maximum track widths - more than likely this is preventing you from making the width bigger.

PADs is illogical because to set track widths you have to use the "clearance" menu and this just does not make sense to me. I'll also add that PADs is the 2nd worst piece of software I've ever used. The worst piece of software I've ever used is also made by mentalgrahics (DxD) so you have my great sympathy!

Or, it may be just you need to start a route then press w - a box will appear containing the "w" and you press spacebar then 50 - this will give you a 50 mil track width (providing it isn't constrained by my previous recommendation about going to the clearance sub menu).

If you want to draw solid copper be prepared to jump thru a few hoops. You need to be in drafting mode (I think it's called) and select the icon at the top of the screen pertaining to copper. Then right click anywhere in the design area and select rectangle. Then draw a rectangle - I think from thereon pads automatically prompts you to select the layer you want it on. If not hold the shift key and click on the box then a right mouse click will allow you to select properties.

Select the layer and then select the net you want to attach it to - there's a pull down menu of all nets and click on the one you want. To move the rectangle use shift left-mouse then CTRL-E.

Have I said how awful PADs is?