[gunsmith_tony]

1, how easy is it to finish a FAL receiver?
If your referring to the "80%" casting...if you dont have the necessary tooling, it can be a hellish nightmare. If you don't have the proper blue-prints, you'll have to reverse-engineer it from a finished Imbel receiver. That too can be very problematic. Needless to say...hand tools won't do it. I have 4-lathes, a mill, 2-drill presses, 2-arbor presses, and 25-yrs experience as a gas-gun builder...and I won't do it.
2, other than the barrel removing tool, is there any other 'special' tools that would be needed?
You can get by with vising the barrel on the flats...assuming you have a "Real" vise. A good receiver wrench however, is a MUST. You will need a set of headspace guages (GO & NoGO). And you will need either a Guage Rod, or a set of Pin Guages with which to measure your headspace in conjunction with the headspace guages. A buttstock tool is also very handy.
3, is it possible/feasible to do this thing?
Sure. As with any project, it depends on how serious and determined you are. The FAL is amung the few service rifles that CAN be built by someone other than a qualified gunsmith. But be warned...don't even try it without the proper tools, and just as important...information.
4, do I still need to replace the required amount of parts with USA built ones? Even though I am the gun manufacturer?
Yes. If your building it up from a parts kit consisting of foreign parts, you'll still have to render it compliant even though you've "Made" the receiver. And since you are not a licensed manufacturer, you'll have to obtain a letter from the ATF certifying that "Your" receiver is a US mfg. part. However, you can easily render it compliant without having to go through all that.