Hi Guys i really want help for this issue !

Ok...so lets start this , long story short, i bought my old 76 duster back, my 'friend" who had it was nice enough to sell it to me but...yes theres a but...he never did anything on the car and beat the crapout of it and really filled the purpose of this car , hes a cop and drove this thing 120-130 mph a couple of times a week and all ( Above the law !! ) Gas tank is leaking..tranny doesnt shift good but oil and color is nice , brakes are due all little nic nacs ...last but not least ****worst thing ****: already the wiring harness was a mess on this car and my old school mecanic back in the day did an ok job , i mean a real temporary job and 15 years later nothing works anymore , no headlights , 1 rear brake light works, and it needs to be completly changed , i didnt have the funds 15 years ago , but i managed to find a complete 1975 318 duster wiring harness from A to Z....people told me that a 76 wasnt the same , can i try the 75 one or hell even buy a 70-71complete and brand new wiring set from year one etc...i cant even drive it like that and it pisses me off and i dont know anything about wiring harnesses and eletrical stuff.