If you just move the cab forward on the frame, you will be cutting the firewall and making room for the motor inside of the cab.
On my 49, I ended up moving the front wheel arches forward about 1 1/2" to match the wheel openings with the tire location. My motor is real close to the firewall with the Dakota firewall in the factory 49 cab location, any forward cab movement will put the motor that far back into the cab. As far as moving the rear fenders on the bed side, there are body lines around the fender, moving the fender more then an inch puts the fender on top of either the front or the rear bed side body lines. I have actually done this stuff, not speculation.
In my opinion, adding 11" to the front of the bed just looks wrong. It makes the bed length in front of the wheels too long. The original rear fender location on the bed puts the rear fenders at the bed center. 11" longer front on the bed is a lot. Most people like to shorten the bed length, not make them longer.
Please don't let my opinion alter your design, if your OK with the longer bed, or altered fender locations,, go for it, its your truck.
Pic 1) Front wheel well arch removed, notice how close the front is to the opening then the rear? that is what 1 1/2" looks like in real life.
Pic 2) This is the best pic I have of the wheel arch welded back in 1 1/2" forward without paint and body work.
Pic 3) The rear fender placement on the bed side. I did not shorten this frame, someone else did. They could have shortened it another 2", it would have been great I know the frame was shortened, but I do not know which Dakota frame they started with. I slid the bed rearward from this position, but there is an inch and a half between the cab and the bed at the center. I still spread the bottom of the fenders and shifted the fenders towards the front 1/2". By the way, the spot welds you see behind the fender are the angle iron that the bed floor sits on, the bed is only about 10" deep.
Pic 4) I think it came out OK.