Is a IRS and sold rear fuel tank the same
Yes.
VN, VP, VR, VS sedans .... all "Physically" the same.
But be aware, there are different fuel pump/sender setups depending on what it came from.
I beleive the VN came with 2 fuel pumps, one internal, one external .... (Confirm?) .... whereas the VS for instance, only has the 1 pump in the tank.
VN and early VP also had a separate return line into the tank.With the later ones, the return line went into the tank through the fuel pump assembly,not through a seperate connector on the tank.
Not all VN's had 2 fuel pumps, I think it was mainly VN's with the long range tank that had 2 pumps. Also the breather lines are different. A VN/VP has 3 connections on the top of the tank, 1 large and 2 small. When I was looking for another tank for my VN at the wreckers some tanks, I think VS or maybe VR/VS only had 2 connections for breather hoses, 1 large and 1 small. The 2 small connections on a VN/VP tank are positioned 1 either side of the large one and are bent in towards the larger one where as the newer ones they are parallel from memory.
The fuel return connector on a VN/VP tank can crack and leak fairly easily if your not gentle getting the hose off. The old tank in mine was leakin gthere when I got it, I just used some of that epoxy repair stuff and it was fine until the tank cracked under one of it's brackets.