The number crunching continues in the Souderton Area School District.   The school board Thursday night approved a preliminary spending plan for the next school year that if left unchanged when the final budget is adopted in May would raise property taxes by about $190 for the average homeowner.   District officials say that number will change though as the budget is trimmed over the next several weeks.   As required by state law, the school board took action on the preliminary budget before the end of January to allow it to seek a property tax increase in excess of the 1.4 % Act One index if necessary.