Can tell you my '89 D100 318 goes with the Y-Pipe heading down the inside passenger frame rail, heading into a muffler just in front of the axle, up over the rear end and out just behind the right rear wheel.
'89 has 2 1/2" pipe from the Y-Pipe back. Your '69 probably used 2" pipe.
That is super helpful...That's what I will have the muffler shop do then. what kind of muffler do you recommend to make it close to original. The parts for these trucks no longer exist.