I spent a bit of time on a 140, trying to make it legal.
I got close by reducing the valve volume by fitting inserts inside it, but the only way I could ensure it stayed below 12ftlbs was to have it so that it needed 14 pumps to get up to power - a relief valve would do the trick.
It's the same with these pistols, although it's generally accepted that you are supposed to put no more than 10 pumps in, if putting more pumps takes it over power then it's illegal. Whatever mods you do you have to test by just pumping and pumping to find the point where the pressure in the pump does not exceed the pressure in the valve and therefore no more power will be made by continuing to pump. I'm very concerned with those flat top piston conversions on the 1377, if it makes full power at 7 pumps what does it do with 10 pumps, 12 pumps. 20 pumps?