all Gen. III/IV PCM's the same?
 

i swapping a 5.3 in from scratch. i have no PCM, no harness. a custom harness is being built but what do or could i use to run the mill? will a legit LS1 brain run it? or do i NEED a truck PCM? are there any difficulties in tweaking any PCM's over any other?

no, there will be slight differences between some years on the tuning for that particular engine

Within GenIII, major difference between '98 LS1
and other years. Also need to know about cable
throttle vs drive-by-wire, many PCMs for electric
throttle omit the IAC drive circuitry. F-body LS1,
early trucks and most vans have cable throttle
and IAC works.

You would want the harness and PCM to be from
the same kind of vehicle, to minimize hassles and
mystery.

There should be no difference in cost, maybe in
local availability. I would recommend you check
with 'wait4me', he has about any kind you'd want
and can set up a "mail order tune" to start with
for however you're building up the motor.