I have a similar problem with a Dahon bike. I tried installing Shimano BR-T4000, but the shoulder depth in the bore hole is only 4mm deep, while the boss shoulder is 6mm high. So there is a 2mm gap. The spring pin goes in the spring hole by around 1.5mm, so it still works. But I'm wondering if there is any detriment in having the gap? Such as risk of breaking the bolt, deflection of the boss when braking?
I had some 2mm thick chainring spacers, and DIN 988 shim washers 10x16x0.1mm thick. So I shaved the spacers to clear the spring pin. I took a pair of scissors and cut a notch in the shim washers. The gap was something like 2.1mm, so I stacked a 2mm spacer and 3x 0.1mm shims for a total of 2.3mm. Then I tightened the bolt to 8Nm. (is this how tight you make it?)
The boss is 5.5mm short of the top of the brake arm pivot surface. The bolt included with the Shimano brakes was shorter than the original bolts on the old brakes, so I reused the old longer bolts.
Edit: I forgot to mention that in the shimano brake bore hole, the transition between 10mm and 8mm is a slight countersunk chamfer, so if you don't use spacers so that the bottom of the pivot touches the frame/fork, then it's a bit unstable and rocks back and forth.
The original brakes were OEM Dahon branded, but looks like Tektro 857AL. The Shimano brakes seem much more stable, smoother, and stronger spring.
Strangely, even with the gap on the Shimano brakes, the pad mount slot is exactly at the same distance from the fork/stay as the old Tektro brakes.
I attached photos and a diagram showing the dimensions.
2mm gap
22.5mm overall length, 6mm high shoulder
boss is threaded but probably with threadlocker. No wrench flats, so I can't safely grip it to adjust the height.
Boss is 5.5mm short of the Shimano brake pivot surface.
Gap closed with 2mm chainring spacer + 3x0.1mm shim washers = 2.3mm
shims and spacer on the boss
shims and spacer cut and shaved to clear the spring pin