I am contemplating purchasing a Nodak upper, either 604 or 603. I currently have a PSA flat top midlength, and was hoping to just swap out the upper, as I do not use any optics and want the old school look. Would the PSA upper components I already have be compatible with either of those uppers? Thanks for your help!

I know I'll have to get a rear sight assembly I was just concerned about the barrel and gas port fitting correctly. Thanks!

They will fit.

Good luck getting one before Easter if then.

I know I'll have to get a rear sight assembly I was just concerned about the barrel and gas port fitting correctly.

All the same spec.  Possibly will fit better.  Figured I'd throw this in due to original question.

If you need help on installing the sight assembly a few decent video's out there but basically only difficulty I've encountered is roll pin for windage drum.  Best to start the roll pin in windage drum before installing over detent and spring.  I use a punch in other side hole to hold while I tap in pin but even paperclip will work to hold them together.  Little pressure on holding punch or extra hand for first couple installs isn't a bad thing as obviously holding tool will back out as pin is installed.  Look at your orientation with or without lining up on drum holes and make sure  you can get to the punch to strike and install pin due to receiver dimension. Might want to do install over a sheet / towel or such as parts are a little on the small side especially if you don't have a bin full of spare A-1 sight parts.

If you are looking to order a M16A1 upper from Nodak. I would order it now. It took me 8 months to get one.

Worth the wait from Nodak if you want retro ...those guys do incredible work. Stuff is first rate