The dash wiring will be quite different. Engine harness should be nearly the same, as would the rear body harness. Headlight/ parking light harness should be nearly the same. I assume you are asking because you happen to have access to a 49 Harness? As long as both are for either a Six cylinder, or an Eight they can be made to work with some lengthening of the dash gauges wires, as the gauges on the 50 are in front of the steering wheel, whereas on the 49 they  are on top center of dash. Who manufactured the harness? YnZ, or Rhode Island? If the harness is a generic one from say Painless Wiring or other, your mileage might vary,,,,,
 

My car has the original wiring harness and I am tempted to order a new one from Rhode Island BUT they only list 1949 Commodore 6 parts. So I am curious if I can just order that, or even I need to pull mine to send in which would be unfortunate since that takes it out of commission for 4 months or until it comes back
 

Here you go, YNZ makes excellent harnesses.
https://www.ynzyesterdaysparts.com/home/wire-harness-catalogs/1917-1974-antique-classic-foreign-domestic-wire-harnesses/hudson-terraplane-wiring-harnesses/

Forget Rhode Island,  get the 1950 Commodore Harness from YnZ Yesterday's Wiring: https://www.ynzyesterdaysparts.com/home/wire-harness-catalogs/1917-1974-antique-classic-foreign-domestic-wire-harnesses/hudson-terraplane-wiring-harnesses/

I put one of YNZ’x harnesses in my 51 Commodore 8. Excellent harness. While your at it install a headlight relay near the radiator support. I’ve also used YNZ’s harness in my 29 Hudson and 34 Plymouth.
I’ve also used RI Wirimgs harnesss in a couple of 33 Terraplanes,,and a 54 Jet. Also very good harnesses.

Current lead time from Rhode Island is  14 weeks..... I've rewired a 50 Commodore from RI's 49 harness. Not a really big deal. 

YnZ wasn’t even on my radar and you all seem to like them a lot, I’ll go check into them. Thanks all!