I agree with Jorge's concept of bleaching, but I think there is an easier way. Simply underexpose by about one third to one half, then develop in soft developer over diluted by about 3-4 times. This allows lith film to become almost as variable as continuous tone film and thin is possible. Do not try to use regular strength and short time here or you will get uneven results, about 2-4 minutes depending on temp and very dilute. Test and see what gives the results you looking for.