Yep, that's the way you measure the wheels b.s.
The one thing to remember is if you're depending upon the air shocks to keep fender clearance, you could be in trouble if they ever give out while on the road and the fenders then rest on the tires. Longer shackles will help to relax the springs a little more when using air shocks since you wouldn't be pulling as hard like you would be with the shorter stock shackles to gain the same amount of lift. Your ride won't be quite as harsh either. On average, the amount of lift a shackle gives you is about half of the total shackle length you add. As an example, a 2" longer eye-to-eye shackle dimension will give you about 1" more fender to tire clearance.

 

I cant remember what the backspacing was on my soft 8's,but if you can find that out,then you should be good to go,like i said mine were 15x8 with the 295/50/15 and after rolling up the fender lip i had just enough clearance for the tire to not rub on anything,it was a very close fit between the fender,leaf spring,and stock type shock,but they did clear.Just seen this same topic on another forum and here is what a post said about fitting 15x10 with the 295/50 on his 78,should be the same for the 81,i didnt have to do this with the 8" rim,but never used 10",so this may help you out.
"i run 295-50-15 on 15x10 with 5.5 bs. use 1/4" spacers. rolled fenders and had to cut the shocks studs to clear the rims."