2003 isuzu/help me understand the os swap

so i have successfully tuned my truck(2004 chevy avalanche 5.3) and i love the results. my other car is a 2003 isuzu ascender(gmt360/debadged trailblazer) with the 5.3. it shows some signs of having a mild cam, but no tune. it has a nice lope to the idle, but no misfire codes and has a bit more valvetrain noise than the truck. off idle it goes lean, but is fine at wot. throws a lean code when outside temps are less than 40 fuel trims are right at the limit just off idle but o2 show 0.9 at wot full load. it definitely has a higher stall converter, that i would assume has also not been tuned for. torque braking against the converter will bring it to 2500rpm before the tires break loose, stock is 1800 ish and breaking the tires loose is unheard of in stock trim on the tb forums. the trans shifts kinda weird, but plenty firm. opening up the exhaust exaggerated these symptoms quite a bit. maybe im wrong and it has other issues, but my research is starting to lean towards this and either way i would like to tune it.

now my problem in tuning it and why i think it didnt get tuned after a cam/stall install. the os in the ecm is 12580055. from what i can find this is somewhere in the chain of oem updates to the bastardchild intel p59 ecm. there are few places around me that do tuning and its very expensive($500 every time they plug in+dyno time) and they dont do os swaps to my knowledge. not a huge deal for me but i can imagine someone else having done a cam after a dead lifter or something, went ahead and did a stall figuring tuning wouldnt be a big deal. then they find out this car they spent way too much to repair cant be tuned, at least not cheap. they then sent it in on trade or let it go to auction. may not have even been the previous owner, could have been a small shop that figured it would be fine. none of this is that far fetched around here. none the less i low have the car and want to tune it. the common solution i have seen is to os swap to the only p59 os that we have a definition file for and not worry about it. now how do i do that?

do i just find a p59 bin that is already optioned for what the car has or is close and do a write cal&os in lsdroid? do i just edit the vin line in tuner pro to the right vin? can someone help me through this? besides os swap what other changes require a write entire?
looking through the bin from the av seems like i am mostly there, but there are some hiccups. one big one is the cooling fan. the avalanche uses the standard viscous clutch cooling fan that has been around since the dawn of time. the isuzu uses an electro-viscous clutch fan, there is a controller on the fan from the ecu it appears to be pwm control and is almost like a variable torque converter, but for the fan. this seems to be a gmt360 exclusive. what option would i chose here? trial and error? similar goes for the vehicle platform option as there is no gmt360 option. any suggestions from anybody? i can upload a bin if needed.