I'm not a FAL guy but regarding improper heat treatment of a receiver,
The item can be warped which on a semi auto makes it difficult to function reliably (see things like rewelded military receivers, etc.) , made brittle (strong like glass but can shatter or crack), or be too soft which means wear is accelerated (won't stand up to recoil). Heat treatment is best left to to professionals with precise controls but for them to successfully do their work--they need to know the composition of the receiver as different alloys require different temperatures etc.
However, my guess is that you have an aftermarket investment cast receiver block rather than a raw forging. Depending on the material used, it may or may not need heat treatment or only need it on specified surfaces. As I said, I am not a FAL guy so I do not know how much stress from operation is placed on specific areas of those receivers.
That being said, your first task is to find out who made the casting/forging which was probably made for the relatively few companies that assembled FAL's or sold receivers for them when they became available. Ideally, there should be some identifying marks that can be used on the receiver so that someone familiar with those receivers could identify it and explain more fully the issues involved in turning it into a working receiver. I would suggest online forums that specialize in the FAL in the DIY sub sections (
http://www.falfiles.com/forums/index.php for example ) might be able to shed more light on the receiver blank that you have. They might also be able to identify jigs, templates, etc. that can help you finish the receiver so that it is safe and reliable in operation.
