I was given a large frame Remington 870 20 ga. receiver. Would it be possible to convert it to a 16 ga. ? Since they are both built on a 12 ga. frame, can I put 16 ga. parts in the receiver and install a 16 ga. barrel ?

I do not think you can...

The frame is the same external size but the ports for barrel and magazine are sized for 20 ga tubes... I THINK!

Besides, wonderful as the 16 is the 20 gauge is better. The lack of 3" 16 gauge means the 3" 20 is just about exactly a 16...

And you can still load ordinary 20s and have a nice light recoiling gun. If your 20 is only 2 3/4" it is easy to convert it to 3" and you can either buy a 3" 20 barrel or have a 2 3/4 lengthened to 3".

I assume you lack the parts to use as is.

Yes all I have is the receiver. After searching the internet,the barrel seems to be very hard to find.

Barrelexchange is a good source for barrels and the owner can answer any questions you have. The barrels for the full size 20 aren't cheap. A standard 20 with bead is about 225 and a vent rib is scarce and around 350.

I have a large frame 20 with a fixed mod bead barrel and it is an awesome little gun. You can always ship me the reciever for parts if it's taking up too much space.

I could help you out with a 2 3/4" Full choke barrel if interested. I have a plain barrel and a couple of vent rib barrels. The plain barrel has a fair bit of bluing rubbed off but the vent ribs have had some superficial rust issues in the past. I have rubbed the spots down but there is still some marring. The plain barrel is a take off from one I bought to use as a slug gun while the vent rib barrels came in a grab bag at an auction.