There are no measurements, if you are just doing one offs
choose your size ,
choose your material
choose your connectors, flange , v-band, or just slip fit with expanded joints...
get some U-J bends of the material you are choosing
and fabricate it to fit...in vehicle.. hardest part is the over the hump bend...
or if you are skillful enough build a jig base on your old exhaust
it's fun, but hope you got the equipment to do it... or the weather is favorable to you..