One of the main fitting problems with sweatshirt jackets is in the shoulder.  The shoulder seam on many sweatshirts does not have enough slope from the neck to the arm.  There should be 1" - 1 1/2" drop from the neck edge to the arm edge of the shoulder seam.  Before you trim, quilt your front and back pieces.  Then pin the shoulder and side seams and "try on" the jacket.  If you have a lot of bunching at the armhole, try pinning up some of the slack at the shoulder/arm edge.  You may need to cut the armhole a bit lower at the sides to balance the opening lost at the shoulder.