Wow, that would be a monster trailer unit. The only advice I can give you is to take your time with the cutting & welding the tanks together, if they are slightly off then you will have problems with the doors, cooking grates & baffle plate alignment.
So upon further investigation, it appears these are 454 litre tanks. (119 gallon) So I think if I were to cut one end of each and weld them together I would end up close to 200 gallon total.
Welcome first of all. Pete can hook you up with some calcs for your pit volume after cutting the bell ends off.
What's the width of the tanks?
If you want bigger look up the MACK. No tank needed.
Here's my work in progress...
My advise is don't settle unless you are in a bind. To build a pit much less a pit and trailer is a lot of work take your time. Plan, plan, plan and recut and recut. It's a roller coaster ride.
RC- I hope she gets painted next week. Painter has been bowed up with the mills annual outage. Then cover will be installed. I have tweaked all I have found to tweak. She's ready for blast and paint.