Given the choice for use on a 60 Dodge town panel between an F,M & J crossmember and a Dakota, I would go with the Dakota. Yes, I have done both.
The FMJ clip was great when it was the only affordable Mopar option back in the day (1992 ish), when I built my 54 Dodge pickup.
The biggest deal with the "F" body clip is you have to fabricate everything forward of the firewall. At the time Tex Smith had a book on using that crossmember on a 53 Dodge pickup, and that book was the authority, but the book was missing important numbers (like the lengths of the tubing) and the info on welding the tubing to the OEM framer was incorrect, the truck sat way too low. The forward front frame rails were 4 pieces of 1/8" wall 2" x 4" tubing. As I remember the deal (it was a long time ago), the 4 mounting points for the crossmember have wider spacing at the front then they do at the back. The rear pair tubes are welded onto the OEM frame at about the firewall. I believe I had to cut the bottom flange off the OEM frame because if I welded the tubing below the bottom flange, the truck sat way too low, and if I inside the OEM frame and above the bottom flange it sat pretty high.. There was a recommended 4" over lap between the OEM frame and that rear tubing. Then the front tubing was added on the outside of the rear tubing with a 4" overlap. Before you welded the tubing to the OEM frame, each piece had to have a notch cut out for the rubber body bushing to sit in, then you had to weld in a "new" bottom on the tubing to hold the rubber bushing, and you had to cut an access hole in the inside so you could bolt the cross member to the finished frame. You also had to make a clearance notch on the outer bottom edge of the tubing to clear the bolted on cross member. Then you had to fabricate a front cross member so you had something support the radiator and to bolt the front bumper to. There was serious modification to the firewall for the motor to clear (my 360 motor sat 6" into the cab and you also had to modify the floor for the steering column. The steering column was a challenge as well because the distance between the firewall and the steering box mount on the cross member is very close.
On the Dakota, were you thinking using the entire Dakota frame, or just the front clip? Someone here has recently clipped a 57 Dodge pickup with a Dakota frame, I would look at that. If you use the entire Dakota frame, the area behind the seat is kicked up above the rear axle, and it will raise the rear floor of your panel about 6" or 7". Probably not want you want to do with the Town Panel.
i have a couple old pictures here of my 54 frame before it went under the cab(most of the pictures went with the truck). if I can get my stuff to cooperate with me. Taking pictures of pictures is always a challenge. The pictures posted are my 54 with the F,M,J clip. For reference, the front edge of the running boards did not clear a 12 oz Pepsi can, and the bottom of the front bumper had some really deep gouges in it. The cross torsion bars could not raise the front end more then an inch, and then that 1" lift didn't last very long before it was back down where it was. If I had it to do over, I would have welded that rear tubing into the OEM frame another inch higher then I did. It was a fast hard lesson about how to drive a very low truck. It drove great (once you got in it) Back then I was still pretty skinny, but the steering wheel (with a tilt) was very close to my belly.. We put 40,000 miles on it before I found something I thought I had to have worse. That 54 pick up is the only previously owned vehicle I would ever considered wanting back.
If I can post the frame pics, I'll add them later.
