Blazer gas tank dimensions?

[Keith in NY]

I have a 64 pickup that I am moving the gas tank from behind the seats to under the bed. I think a early Blazer tank will fit, but would like some better dimensions to see if I need to cut the frame or crossmembers. Everyone I have called with the tanks has no idea if the dimensions are the tank, or the tank including the flanges

Could you help a guy out and take a tape measure to your tank? Wondering what the size of a 20-21 gallon tank is. With or without the flanges.

Thanks in advance,

Keith