There's a big dent in the side of my current tank and the bolts have been sheared off the filler cap housing, so I picked up another tank. I got it cheap, so I can sell it on if it doesn't fit without losing any money. The guy said it would fit a K75, but I think it might actually be from a K100. Now I have the tank next to the bike, I can see that the one on the bike doesn't have a big rectangular tab on the back of the tank, and the replacement one does.
This is the gas tank on my 1986 K75C with no rear mount:

This is the replacement tank with a rear mount:

Before I tear into the bike, is there a chance in hell it's going to fit or can be made to fit without welding? Thanks!