I'm a little rusty on plymouth E-bodies, but, I'll try my hand at this:
Are the rear valance cut outs a 'cuda thing or not?
Generally yes, but there were plain Barracudas that got them.
Are there 'Cuda's with 318 engines? Or do they come only with BB?
From 70-71 the Base 'cuda engine was 383 4bbl. 340, 440, and hemi engines were optional. In 72, the 'cuda package could include a 318 or 340. In 74 they replaced the 340 with the 360. Also of note, in these years 72-74, you could get the 340/360 in the Barracuda.
So is the 'Cuda the performance line like the R/T is for the Charger?
Yes
So what makes a 'Cuda a 'Cuda?
Second digit of the VIN. BH is base Barracuda, BP is Gran Coupe and BS is 'cuda
Can you say 'Cuda to a Barracuda with the 'Cuda option package?
Little
CB
I guess so, 'cuda was used from 64-68 before it became an option/model on the Barracuda.
oh, and for 69, 'cudas only had 340s, 383s, and 440s, was a special package with a BH VIN (just like the normal Barracuda)