Which frame rail connectors to use?

The ones that contour to the floorpan add a significantly higher level of rigidity to the unibody. With the connector tied to the floor, you are using the entire floor plane as a stucture.

Are the standard style connectors better than nothing, yes. Better than the contoured ones, no. Can't really go wrong with anything that help rigidity of the unibody.

I built some out of 2x3 .125 tube. I have a set of US car tools for a Duster as well. If you don't want to cut your floorpans, the 2x3 tube route is not for you.