360 oil pan problem

yesterday i got my pick up tube and decided too swap out my oil pan when i went to put it on i noticed that the rear of the pan was nowhere close to sealing about 3/4 to an inch gap whats the deal with this? my 360 is from a 88 or 89 ramcharger it has a casting date of 12/12/88 as for my oil pan was told it was for a 360 theres a number on the pan 822, is a rear main seal area different between yrs of 360's? will my 318 pan interchange instead ?the 318 is a 73 out of a challenger or do i put the truck pan back on it if so will it fit in my dart?

Thanks Terry