1987 Dodge Ramcharger DUAL DRIVE BELTS SLIP CONTINUALLY

Why do my drive belts continually slip. Every time we replace them and tighten them within the 1/4" tolerance (we never tighten the belts to tight as to damage the water pump etc) the belts slip within driving a hundred miles or more. When I start the Ramcharger the belts squeal almost immediately! Also a problem that we have run into these days is auto parts dealers not knowing what matched set of belts means. The numbers on the belts match but they may differ in length as much as 1/2" to 3/4". Matched belts used to come tied together because they were tested to be exact. This seems to make a difference with this Ramcharger. Are there some special heavy duty belts especially for the Ramcharger or where do I go for the correct belts or is this not the problem. What do we do to address this simple but baffling problem that has become a major pain in the butt and drives me crazy worrying when the belts are going to burn up?
Carlton Lee Harbaugh