I have a RM250 T pipe that will be going on a PE250. The basic pipe dimensions are all the same till you get back where the muffler goes. There is a few different little bends in that are because the 2 use different mufflers. You have sold a GMC RM 250 T pipe to a a guy in UK with a PE250 and from memory it basically went on find and he used his own muffler. Basically if this guys wants a pipe that will go on withno mods then the pipe needs to be purpose built for the bike and the muffler used, BUT the RM pipe will go on but may requre a few mods or changing the last 200mm or so of the tail pipe.
Basically RM250 79/80 frames are dimensionally the same as 80-82 PE250. The major differences are the RM doesnt have the rear frame loop and the left center down tube is a bit different on the PE to provide and area for the battery to go, but other than that the frames can be considered the same.
And if you want your RM pipe to fit the PE 100% perfectly, then you just need to get hold of a 80-82 PE 250 pipe and copy the last 200mm or so and add that to your existing RM design.