can this be done legally?

If you have a virgin receiver and sent the plans to the BATFE for approval, and it is approved, yes.

if i have good plans is this likely?

Yes, if good plans happen to separate it from machinegun status and you haven't pissed the BATFE off recently.

i plan on getting a piece of square steel tube for a handle and cut a barrel to 3.5 inches (or whatever a half inch over legal is). must it now remain a pistol or can i switch it back and forth?

You need a virgin receiver, first of all. Secondly, there is no legal federal limit on the barrel length of a pistol. As long as your receiver was never registered at factory as a rifle, or longarm on a 4473, you can make it a pistol. You can also attach a stock and go from pistol to rifle at will, however, you must maintain an overall length and barrel length that remain within legal rifle limits, otherwise you will need to get BATFE approval and register the weapon as either a Short Rifle, or Short Barrel Rifle appropriately.

is it required to be 21 to buy a virgin receiver?

No. You may be prohibited from manufacturing a pistol if you are too young to own one in your state, though. If you are able to own a handgun at 18, or possess one, in your state, you should be legally able to manufacture a handgun regardless of whether you are allowed to purchase a handgun from a FFL. If you live in a state with an AWB, such a handgun will most likely be banned from manufacture and/or ownership.

i live in new york. what i dont get is that i can buy an ak 47 but i cant get a .22 revolver

My advice: buy the AK. Edit- And move. What are you doing in that awful state anyway?