link to mustang gas tank install

[legotech7]

Ok,I am stuck on my tank install and I really could use some help, as I may have made the wrong choice in choosing this tank. Please bear with me as I explain my problem.After doing the research and taking all the measurements I thought I needed to take, I opted for this tank, a mustang tank ( 65 to 68 ) from tanks inc. This tank has been installed in other chevy frames with some modifications.
http://www.tanksinc.com/cartimages/prod_zm_311.jpg The measurements for this tank are 32-3/4''x24-1/2''x7-1/2'' deep..

This is what was in there before, the blazer tank.. pic#1

As I position the tank to try to fit between the frame rails, I see that it won't squeeze in at all. the measurements for the in side of the frame is 33-1/2'', the bottom side of the mustang tank is 29'' even from end to end.Pic#2 new tank up against the frame

In pic # 3, this is where I got stuck..The bottom part of the frame that you see is 3'' wide. In order for me to slide the tank between the frame I would have to cut approx. 1-1/2'' from each side. That seems to be a bit too much I think with out loosing any strength integrity of the frame. Correct?

In pic #4 The top part of the frame towards the front is 3'' wide and tappers down to 2-1/2'' towards the rear, but the bottom portion of the frame is 3'' even front to rear.

I don't know if its safe to cut off from the bottom portion-of the frame as I mentioned before with out making it weak..

I don't know what exactly I need to do here and could use some help or advice. I was told from the guy's at Tanks that people use this tank for a lot of under bed conversions with minimal issues. Is there something I can do to get this tank to work here? I'm not even sure if I can return it. This is a real headache right now.

Thanks everyone for any help you can offer. Sorry for the lengthy post, but thought it was necessary.......