frame notching

[TXsilverado]

so, I have the tools to do the job, but I'M not experienced in this area at all.

I need to notch the crossmember on my 69 Camaro to make room for the alternator. before anyone asks, no I cannot use different accessories.



the engine is currently bolted down, and it's sitting on the front tires. my questions:

1) do I need to get it off the front tires before I start cutting, or can I cut out the material while it sits on the ground?
2)what thickness metal should I use to box it in after cutting out the material I need out of the way?
3) any pro tips/tricks I should consider before I start hacking at my crossmember?

i'm looking to do something like this: