Mont Switzer did an article in Mainline Modeler August 2004 detailing the kitbashing of a Mississippi Central 12 panel early PS-1. If you can obtain a copy of that article I think you will have a good start to your RI PS-1. Also see Ted Culottes article Essential Freight Cars 40 Early PS-1's RMC Jan 08 P94
The early PS-1's had what I call a 10-2 panel roof the end panels didn't have the raised panels, the end ladders were straight, they didn't have the raised tabs under the running boards on the ends, the grabs on the end were across the lower two ribs, the side tabs were different. the two end panels were rivetted not welded etc.
Your best bet is to start with an Intermountain undec PS-1 kit.
There was a very good article in one of the train magazines detailing an early PS-1 but at this moment I can't find it. AS soon as I do I will forward the info.