if your brake booster on your 81 is an M90, i would go that route, use the 83 BB and BMC.  
  
i used an 82 BB on my 83. 
  
get everything you can if your not 100% 
  
Brake booster, brake master cylinder, both brake lines that come off the BMC, brake prop valve, 3way split, and the brake booster check valve.