Besides the diameter, what is the difference? Also, will having the tips pointing at the ground have any negative effects or will it still sound good?

 

the 2.25" will be good for the low end torque, and 2.5" would be more the mid and upper end. depends on the engine and how much power it's making. a 500hp 460 like my buddies would use 3" because 2.5" at that point would start to be too little exhaust for it.

And if they are turned towards the ground under the bed it will amplify the sound. If you have them exit like the picture though, it should be far enough back to not produce too much noise in the cab.

 

I should probably go with 2.25'' then.

 

okay so you'd want this then
http://www.summitracing.com/parts/FLO-9425452/

this is a tip similar to the two in the picture
http://www.summitracing.com/parts/IMM-577594/
it is the right inlet diameter, you decide how long you'd want it and the tip outlet diameter

you can just have them weld this muffler in, and make everything after that. If you want to come back and swap out the y-pipe after the cats you can get one from magnaflow and take it back to the shop and have them put it on for you.

 

If I understand you correctly, I don't have to order a y-pipe?

 

nope, just what i've read on hear, the stock y-pipe is restrictive, and replacing it only helps. not necessary though.