Marty
I'm no expert and new to the board but I do have a small 45 acre tree farm on my place and use the Division of Forestry regularly for help.
My last planting of Southern Yellow Pine was a 3 acre strip 60' wide that runs beside my drive. It was recommended that I shoot for 6x8 plantings like Ron recommended but initially to make them 3x8 or 4x8.
After about 4-6 years thin them to the desired 6'. The thinnings are throw-aways and only used to get the trees to grow up rather than out. This promotes a straight stem and minimizes limbs. What you are looking for is a tree approx 8'-10'ft tall that, when left from the thinning will canopy over the remaining gap and continue to promote self-pruning.
12 to 15 years I'm to thin again. I'm told that the easiest way is to take out every other or every third row depending on how good they are growing. This wood is pulp.
At 20-25 years thin again by taking out every other tree. Taking out the rows earlier gives a feller/buncher room to do this without hurting the remaining stand. These trees should be big enough for chip and saw.
The remaining stand has enough room to produce saw timber, poles and veneer. (if I can live that long)
I already anticipate that last thinning because I can picture those big, tall, trees on 12X16 Centers and my wife (the mowing nut) making it look like a park.
Rather than use Roundup why don't you try to find a chemical called Arsenal. Arsenal is used here to control broadleafs and doesn't hurt pine. It is used for "release" programs and sprayed indiscriminantly on the entire field. It works by inhibiting the production of a protein that is only found in plants and was marketed as being safe to animal life because we don't have that protein in our system.
I have used it for injecting and got really good results although it works quite slowly. A tree will be dead a long time before it realizes that it is. That is good too if you have a secondary market for the hardwoods like firewood or pallet material. The wood is usable but the stump will be dead and won't sprout back.