I'm doing a ca cOnversion and can get a Toyota hiace diff that's already been cut down and shortened the thing is they are coil how hard would it be to convert it ?

Too hard unless you go coilovers. Even then it's still too hard.
Fold up some flat plate and make leaf spring mounts.

Convert the diff to leaf spring. Pretty easy, just buy weld on leaf spring mounts and remove all other brackets.
Use something like http://www.kustombitz.com.au/c/407671 ... -spring-diff-hangers.html
Make sure the diff is centred and pinion angle is correct.
Convert the car to coil springs, pretty hard as dattodude said.

cut the seat off for the coils and weld some locaters for leafs on. not hard at all