I can`t seem to be able to get anyone including my salesman to measure the bed for me to see what kind of extra gas tank I can put in there . (width)
There are no megacabs in my area.
I need the front of the bed (against the cab) to the wheel wells .
I had a friend measure a regular cab and he says 8", that doesn`t seem right .

Anyone have a megacab RAM?

My buddy just bought a new one (2021), I'll look at his today if the truck is there. One more "glitch" with your plan, my buddy's is a mega-long bed 2500 with the bed boxes in the fenders. I think those make the bed narrower than a truck without the bed boxes. It is a nice truck, his was like 80K MSRP, interior was beautiful. He traded out of a 15+ year old duramax crew 2500 long bed that was very very tired, well used.

We have one at the farm I can get a measurement later today.

Jupiter they make a mega long bed? SRW?

There are companies that will custom mod one for you.

http://stretchmytruck.com/services/longbed-conversion/

mega-cab-long-bed-conversion.html

The one here is about 3 yrs old and it’s a mega cab short bed.
16” between fender well and cab.

2019 Ram 2500 megacab with 6.5' bed. You've got 16" of useable space there max.

Perfect , thanx guys!!

okay sorry one more , how high and still use a tenneau cover?

How about a Ford instead? My buddy will have this in stock next week.