Mustang5090

Hey guys im doing my trans in my 1990 mustang Lx 5.0 I got the trans out today and it looks like a good time to remove the smog pump due to easier acces to the back of the block haha The only question i have is what belt length should i use and how should i route it? I do have underdrive pulleys all your help would be greatly appreciated and if this is in the wrong place sorry first time ive posted on here