I am rewiring a 72 SS with guages, 350 with A/C.. Have the correct underdash harness and A/C harness.. But the 2 under hood harnesses which one has the guage connections in it. The forward lamp side or the engine side. and what about the tach wire. which harness has that one..

Tach wire should be a brown one coming from your bulkhead under the master cylinder.

Got this from Elree.
72 forward harness

If you look closely at the wire side of the dash harness bulkhead connector (separate the fuse block from the bulkhead connector) You should see letters marking the rows and columns. The rows are A,B,C and D. The columns are S,T,U and V for the forward lamp harness and W,X,Y and Z for the engine harness. The following instructions will refer to these coordinates.

Forward lamp harness
AS – No wire
BS – Tan wire (for brake warning light) -From DU
CS – Light Blue wire (for left hand direction signal) NO CHANGE
DS – Yellow wire (for windshield wiper power) -From CY
AT – Black wire (ground from steering wheel horn button for horn relay) -NO CHANGE
BT – Brown/white wire (from ignition switch to voltage regulator) -From CU. --Two wires on light harness. The second, Brown is from the GEN light.
CT – Dark Blue wire (for right hand direction signal) NO CHANGE
DT – Light Blue wire (for windshield wiper) -From DY
AU – Light Green wire (for headlights high beam) NO CHANGE
BU – Brown wire (for running lights) -From BU
CU – Dark blue wire (for windshield wiper) -From CZ
DU – Black wire (for windshield wiper) -From DZ
AV – Tan wire (for headlight low beam) NO CHANGE
DV – No wire

Engine harness
AW – No wire (engine harness has black wire to trans switch for TCS)
DW – Red wire (main power feed from horn relay/starter) -From DV
AX – No wire (Would be a tan wire for TCS)
BX – Pink wire (connects to a brown or white/orange/ppl resistance wire to coil +) NO CHANGE
CX – Black/White wire (for amp gauge) -From DT --No wire on light dash harness
DX – Black wire (for amp gauge) -From DS--No wire on light dash harness.
AY – Brown wire (for tachometer to coil -) --No wire on light dash harness. On the 71 gauge dash harness this wire passes through the firewall.
BY – Dark Blue wire (for oil pressure switch) NO CHANGE
CY – No wire
DY – No wire
AZ – Purple wire (to starter solenoid) -From AW
BZ – Orange wire (for heater blower not used on air conditioned car) NO CHANGE
CZ – Dark Green wire (for engine water temp sender) -From BS --Temp switch on light harness.
DZ – No wire (would be dark blue for TCS
solenoid)