Help! I have no speedometer

Ok so my spedo was working great untill we just swapped out the 318 for a 360 we used the 904 that was on the 318 but now the spedo dosent work we have checked the spedo cabel to the trans and to the dash and neither are the problem, so what would it be? Car is a 1974 dart swinger 360/904/8.25 rear if that helps at all please help guys thanks