I've got the 44mm hx30 over a hx35 12cm. Works great, boost is near instant, basically I can make boost below usable engine rpm.
Head studs are necessary for sure but I've been running 40 psi and up for 125k, 9 years. I really don't think a 4bt needs o ring at 40, 60 and up might be a good idea tho. No idea about the Isuzu engines tho
I've seen the powerstroke engines, turbos are not huge, I'm sure they will work but they might be hard to come by these days. 2 genuine holsets will set u back under 3k, proven combo