Where your bass hangs is going to be VERY important with your narrow string spacing. Try to keep your thumb as parallel as possible to the strings to avoid smiting the wrong one. If that involves hiking your bass up a bit, give it a try to see if it helps. Also, if I know I'm going into a slap-heavy section or song, I'll give the body a quick lift to get the neck closer to parallel with the floor. This slides my strap a little and repositions the neck and strings to be more parallel. I find it easier to get my thumb into the correct position for me that way. Hope that helps, if you still feel that it is a PITA, try a four-banger at a store and see if you feel more comfortable. I usually get funkier on a four, but my fives are really comfy now too, so it doesn't seem to make as much difference now.
okay okay, so parallel is the goal.... i do wear my bass pretty high, i have found this helps a lot with slap and finger technique. we'll see... my entire mindset is that it takes a long time to get "COMFORTABLE" with anything... and i just have not put the time in with slap technique. hopefully when i put the time in, my comfort level will increase and i'll be able to rock out them lines!