The front. If a U-joint fails in the back, it wont hurt much if it drags the ground from the rear, or slips out. If it breaks in the front, you could be catapulted and flipped back over front or have the rear ripped out, or???????.
Okay just a bit of DUH factor on my part. Makes sense on the front loop. I lost a drive shaft once but it was from the rear whereas it tore out the ubolts and left my drive shaft going clang clang out from under the car. Wheel hop did it.
My kid brother "tried" both ends on different occasions (not on a my cougar, thank God) and, when broken in the front, drive shaft punched upwards through the back instead of flipping the car. The idea of having a failsafe in the back is not so bad... breaking off at the back, with the spin factor, that thing really bangs around, doing damage.
According to the rulebook, only a front loop is required, but many guys will add another one at the ladderbar/4-link rear suspension crossmember just so the car doesn't get beat up if the rear joint or pinion yoke breaks. NHRA doesn't care about that, just the safety issue with pole-vaulting. One downside to the two loops is it makes removing the driveshaft a PITA!
I didn't have a rear loop in my old Camaro. Just finished the car. Second pass after 'back-halfing' and the 9" Ford pinion yoke broke at the 1-2 shift. The driveshaft beat the heck out of the brand new bead-rolled aluminum floor (and got bent from hitting pavement). BTW, a 9" housing from a 71-3 Mustang/Cougar bolts right into a 70-81 Camaro... With a longer pinion flange from an Econoline, you don't even have to change the stock Camaro driveshaft!