Sure!!! I know where there are lots of 71-74 b bodies. That area of the frame rail might have the same angle. I will keep you posted.
71-74 B body rear rails are different. Only the front frame rails are the same between the 70-74 E and the 71/72 B bodies. 73-74 B front rails are different as well because of the isolated K frame mounts.
AND, don't section the replacement rails. If the piece you're looking at is rotted, how far behind do you reasonably think the rest of it is? Welding good steel to old, corroded ****e is a recipe for disappointment. Put the whole rail in and never worry about it again- and that's my 
Sectioning the rails is not a problem at all as long as you take the sections back to good metal. Depending on what needs to be replaced and where the rust is, sectioning the rails can save you a lot of time and headaches. For example, if you don't have to mess with the rail at the shock mount crossmember, it will save you a ton of time and make it easier to maintain the frame measurements. Been there, done that.
I sectioned both of the rear rails on my Challenger. On the passenger side I just bought a replacement section, Sherman 16082-R. It's a 35" rear section that replaces the rail from the shock crossmember back and has a section that slides inside the old frame rail. Much easier to deal with, the only thing it actually locates is the rear shackle mount. And leaving the shock crossmember intact saves a bunch of time.
On the driver's side I found that the rail was soft further back than that. I bought part of a frame rail second hand, a member here had to buy a whole rear rail just to fix the first little section of the rear rail so I bought the leftover, which was the rest of the rail starting at about the sway bar mount. That goes through the shock crossmember. I added a couple of gusset plates on the inside to slide inside the old rail and went from there. In this case it would have been easier to just replace the whole rail. I didn't need to, the front part of the rail was solid, but it would have been easier to just spot weld the new rail in. But I definitely saved some money buying the leftover part and it replaced everything I needed it to. The install took more than twice the amount of time though, pretty much all centered around the work I had to do at the crossmember. The sway bar bracket also provided a nice gusset on this one.
Take lots of measurements!!!
