The valve seats and all have a "secret coating" for tighter tolerances. They cant be polished. There are no valves available to do a regrind. All this and a few other things like a super thin cylinder liner also makes later N'*'* UNrebuildable. (Ya I see some BS rebuilders offer N*'*) Good Luck with them) To do so will ruin them. The PCM is engineered for the heads that are on it. Not even sure that a change would help if if could be "tuned in". The LD8 isn't really detuned. It makes 300 Lbs of torque out of it'* designed 275 HP. The relationship of HP and resulting torque are often misunderstood. Here'* the specs per GM
high-output 300 hp (224 kW) and 295 ft·lbf (400 N·m)
high-torque 275 hp (205 kW) and 300 ft·lbf (407 N·m)
So what do you think your accomplishing by putting L37 parts or heads on? The LD8 is NOT detuned. It is RETUNED to deliver faster, and more torque. Mix/matching parts between the two engines isn't going to improve anything. The LD8 quite simply makes better use of HP and with a much more efficient power curve. Alot of people have been fooled by an old Admins here who hasn't a clue about engine dynamics. The L37 is not a superior engine. It is one designed for highway, not performance driving.
To that I will add the 04 LD8 came out with some newer and more sensors. Harness'* not likely to match up or work in our locked PCM scheme of things. And we can add taking the heads of L37/LD8'* is something GM Techs avoid like the plauge. The odds are you end up buying a vey expensive rezert kit, and good luck in getting things drilled out and put back together in a way that doesn't lead to a blown head gasket or warped/cracked head.
How'* that for some input???
Everything I mentioned here has been covered in this Section in the past and I feel like a "parrot"
Drive your GXP as is and enjoy. You can Nitrous or mild turbo. All the moderate choices available after 7 years of aggregate brainstorming.