The mounting points on the left are for the front of the bed. The one on the right is the gas tank support. There should also be a pair of bed mounting points just to the back side of the gas tank support.
What are you doing with the bed floor, making a wood floor, or using the cut down Dakota steel floor? For that matter, are you keeping the Dakota fuel tank?

Often times, the bed mounting points on the left side of your picture will sit behind the "new" cab and may be positioned in about the correct location for the front bed cross member, you will want to check that. If the front bed mount bracket is used, the standard front cross member on the Dakota's steel bed floor lifts the floor high enough to clear the fuel tank's frame mounted cross member. If your using the Dakota fuel tank, there is enough leeway for the front tank strap that tank cross member can be fudged towards the rear up to an inch or so for additional clearance on the bed cross members.

When I built the 39 Dodge, I ended up removing the front bed mounts on the frame because there was too much space between the cab and the front of the bed. I also was using an 8' Dakota bed so I bolted the bed floor to the frame using the 2 pairs of the center most bed mounting points, and then shortened both ends of the Dakota bed and made new brackets. As long as what ever your using for a floor in your bed clears the top of that fuel tank frame mounted cross member, its probably easier to make new brackets for your front bed mounting bracket.

By the way, if your using the Dakota steel floor, and a rear axle not from a Dakota, you will need to add a spacer about 2" wide to both sides of the Dakota bed floor where you cut the wheel wells out. And, if your using the Dakota rear axle, your going to have to mini tub your 50s' step side bed sides, they won't clear the rear tires. Gene