Will a 73 B Body Rallye gage Cluster & Harness with fuse block fit a 71 B Body? Are there any issues to address if so what?

NachoRT74 should be along soon to fill you in on all the details but yes you can use it.

well as stated some differences comes with them:

-Some wires were turned around along the 71 to 74 years at bulkhead conector... Horn, wiper washer, Brake proportioning valve, starter relay are mostly of them. Take care of match the 73 underdash harness with your 71 engine bay harnesses, check every wire!

-on 72/73 you get seatbelt relay and sensors ( seats and retractors ) to control warning light and buzzer for a while IF OPTIONED on 71s they were flasher time relay controled ( same that controls the ign courtesy light )

-with seatbelt warning system, the NSS ground is sent to inside the cab to 73s ( brown traced yellow ) to make it work just when engaged, for a while 71 don't. that wire uses one of the cavities of the tranny plug at bulkhead. Those cavities were used on AC cars for clutch and blower ( blue and green ) on 71s, for a while 72/73s used any cavities available around on bulkhead for those. Normally the other tranny plug cavity and an engine harness plug empty cavity

-Rest of plugs and wires for seatbelts and relays can be left around inside the cab without use to install on 71s. Won't affect anything

-Seatbelt lights are controlled diff between 71 and 72/74. If you have a 71 car with Driver Convenience package ( low fuel warning indicator ) will need a further job to make it work correctly on a 73 setup. 2 options for that that I can detail if needed.

-AC/Heater control... totally diff. Use the 73 OR use the 71 but matching blower lever plug and wires location. Be also sure about the control switch wires location. 71/72s turns off in the middle, for a while 73/74 turns off at a side. Allwas has been told the control switch is the same, but being they turn off in diff position thats something to check, between the switch and wires arriving to it.

-wiper plug won't match although wires are the same. If you are changing from standart to rallye, will need to keep the 73 wiper switch and use the matching harness/plug. If you are changing from 71 rallye to 73 rallye because some damages on your old harness and cluster, then take care about what wiper switch you will use and match the plug(s) with switch

thats all I can remember right now... sorry maybe to much information, but if some extra needed I can answer it more detailed

Yes, I agree with all of the above. I went from 74 standard to 71 ralleye. I had trouble with the seat belt wires, I just ended up passing it through, and ignoring it all together. I also absolutely ran fusable links to the alt gauge of course. Also the brown and blue wires for ignition run were opposing on the two years on the steering column when I matched them up, the wiper switch gave me no trouble, ended having 3 speed, went right in. Next, blower controls, yes you wil need 71 controls, or to switch as he said. It was farely simple using the wiring diagram to complete, do yoursel a favor and double check everything, I friend the ingnition, because I had one open wire after I installed it, dont make my mistake...

ouch... 74 is harder to match CORRECTLY.

I converted one 74 into 73 once ( with some 71 specs too ), removing al the interlock system and attaching some of the standart harness from the 73 harness at the right points, removing from end to end everyone, replacing them with terminals included to not cut any wire as far was posible

then when I got completed the 74 interlock stuff, got it back installed so made the reverse operation.

That it was because Venezuelan cars didn't got the interlock deal and I wnted to install it to make a correct USA 74 Charger