Went to my local bone yard and got the interior wiring harness from a camaro complete. I also got the ecm out of a 91 firebird code E 305 tbi, as well as its complete uncut engine wiring harness, plus its vehicle speed sensor for a total of $45.

My question is that I saw some differences between both engine wiring harnesses.
1. The new engine wiring harness has a box with 2 connections that was mounted behind the master cylinder. Will this interfere with my setup??

2. Some connections are different. Such as AC compressor pigtail, throttle body pigtail(front), and the holes on the fusible links to starter are smaller.???

Also noticed 7 grounds behind the motor to this new harness where as the old had only 6?

 

Need a pic of the "box behind the master cylinder"...
A/C compressor pigtail will probably swap over, but I don't see where it could be different as the connectors were the same on my 87 and 91 harnesses (yes, both of the 91 harnesses). The others will probably need to be swapped too.
The extra ground is for the SIR airbag system you don't have. You can use this wire for any add-ons.
You may need to swap over your wiper motor wiring as in the later cars it's in the front lighting harness and it's in the engine harness in the others. Check first.

 

wherent the 91s Speed density instead of a MAF setup??? thats they only difference that I can think of

 

i think one of the boxes he's referring to is the headlight module for the firebirds (since he's got a camaro now and that's something not found under those hoods)

 

Yes but that applies to TPI & V6. TBI should have always been SD.

The 91 harnesses & ECM will be setup for VSS & VATS. 2 things that made me not swap a 90 drivetrain into an 88 car.