First -- it would be nice to hit the "enter" buttons once or twice, it does help us readers!
Second -- Did you notice a lot of gunk/dirt/brake pad dust in the cavities of the ST-40? Might be worth an hour of your time, tops, to pull off the calipers and inspect/clean and retry installing them.
From personal experience, on my ST-60/40 kit I didn't have any issues like you are reporting, front or back. But do recall once when I threw on a new set of pads, they were about 85-95% of the way seated in the caliper but that last 5-15% took some force to push. I took them back out, pull the guides/metal retainers and cleaned all around. Re-installed and boom! no issues after that. They do take a little wiggling back and forth sometimes however its always by hand.
How thick is the coating on your backing plates? If cleaning out the caliper doesn't work, consider taking a file and lightly filing once end of the pads backing plate enough to get them to go in by hand. I only say that because you'll scrape off the finish on the caliper.