Longer shackle raises teh body above the spring, but also leverages the weight at the most advantage over the spring.
The back half of the spring essentially supports the car's weight.
Re-arching has worked for me in the past. Years back, about $125-$150 from a good shop.
Where I live now, there's 1 spring guy within a 100-mile or so radius, and he's greedy: $400-$450 to re-arch.
I bought springs from Espo for much less than that - about $280 to my door - and I'm very happy with them.