I had no idea there still existed confusion on belts and tensioners after all these years.
1) belts don't come U shaped. They get that way from spinning/slipping, misalignment and/or old rubber stuck in the grooves.
2) Gates belts are best, and the green stripe is their HP belt. The totally green belts are their extreme duty belts. Gates makes belts for Napa and others.
3) Being flat on top of the grooves is not a bad thing, in fact it's preferred as it puts more load where it can be of the most benefit - the sides of the V.
4) Due to the large variety of pulley sizes and other variations in hardware on people's cars, it is impossible to just say xxx will work great for you. YRWV.
5) Re-clocking the tensioner is of minimal benefit. Yes, it increases tension, but if you already have MP springs, there is no need to clock the tensioner. If you DO clock the tensioner, you have be aware of the max limit of travel with respect to the belt. If the max travel of the tensioner is enough to allow the belt to touch itself when it is at full extension, then = fail. Test this by using a belt that is way too long and just pulling it tight with your hands. If the belt touches itself at this point then your re-clocking went to far and you are just going to chew up belts.
6) Pay close attention to the travel limiter on the tensioner itself. Many people make the mistake of swapping the upper and lower tensioners. The two tensioners are the same but the difference is the limiter. The bottom tensioner limits earlier than the top one. If you swap them you will find that the tensioner reaches it's travel limiter way early. Regardless, if you don't have a good 3/8" of travel to the limiter tab then it also = fail. I suspect this is your #1 problem. The tensioner is probably reaching its limit due to belt stretch and unloading the belt. As I said, the belt stretches more than you think.
7) If you have checked everything and you simply cannot get enough travel without running the belt into itself then you may have to raise the blower slightly. Due to differences in pulley combo's I have found that sometimes this is necessary with 10 and 15% combo's. With the MP 15% 10 rib kit this is mandatory.
In any case, there is no excuse for belt slip on an M90 at less than 25% OD - without blasting or modifying the pulleys.
Well, there is one excuse - if you have aluminum pulleys and have been slipping them for years then it is possible to have worn out the pulleys in which case blasting them won't really help.