Diesel Bombers - 6.5 exhaust

hi, i joined here a year ago and just haven't done much here. i'm hoping to get more involved and as my diesel gets older, i'll be looking for advice on repairs, upgrades, which new diesel to buy...
right now my 98 6.5 has an exhaust leak off the left side manifold. the bolts look like they are not going to let go easily. i'm considering an exhaust upgrade, something to help it breathe. i already installed a k&n intake kit, and as i research exhaust systems i'm bewildered at how many there are, all claiming to be the best. so, does anyone have any advice for this? i have the intake installed, if i go to a bigger exhaust, will i have to change anything else? will the new exhaust help keep engine temps down during towing? anything else i should be aware of?