I use 1" narrowed IRS arms on the new speedster chassis, and Kevin from Cool Rides is the one who makes them for us. He does a nice job on them, but I know cores seem hard to come by lately, so we've been supplying our own and hoarding more for future use.
One note on the discs is that you cannot use the forged caliper bracket with his modifications, you have to use the stamped sheet metal bracket version of the rear disc kit.
We make our own a-arm front suspension, but it is specific to our tube chassis and not suitable for a conversion.
Dune buggies have been making tube trailing arms as well as boxed trailing arms...2" and 3" longer.
Not sure why this is even an issue these days, no chores needed. I might be pulling out the table and setting up some jigs. Man I dont have time....but could be of interest...just the rear arms. I will ponder, get into my 3D and see.
As far as the disc caliper mount, my guess is it has to do with welds and or material build up on the newly narrowed arms. Thanks for the heads up.
Interesting in your arms. Thanks. I will have a gander and check them out....