Rear suspension

As I plug slowly along on my 67 Dart, I was talking to a friend and he started suggesting that I change the complete rear suspension out for either Ladder Bars or 4 Link. I started looking on this site and either I am using the wrong search criteria or there is not a lot about changing the rear stuff out. With my rear leafs in a unaltered location I can get 265X18's with about 1/2 inch clearance to the spring and just slightly more to the fender (inner fender well removed for mini tubing). This conversation started because I said I felt my springs were too weak and I was either going to replace them with a new set of CalTrac split Mono springs (these came with the car) or order a set of SS's. I am almost clueless when it comes to suspension stuff and have no idea what ladder bars or 4 links are except it looks like it goes to coil springs. I don't see any issue with leafs. Am I off base here do I need to be looking doing something else or is that just spending money that doesn't need to be spent. The car is a 360 north of 400 hp and will be used 99% as a street touring car. It might see a drag track once just to see what it run's like.