Some very good questions you are asking. Much of the answers will depend on your framework; ie. frame strength and how many perlins and their placement. I say this because it is much simpler to just place hangers on the lengthwise running perlins at whatever spacing you want and then drop a stringline from the hangers. I'd even invest in a twine wrap hanger to go in between. It allows you to easily lower your plants as the season progresses.

The majority of tunnel growers that I've seen with that wide a structure will hang suspended pipes from each arch across the greenhouse and situate rows below these pipes. I run all my rows lengthwise but it has required me to install extra rows of perlins. I support two rows of tomatoes from each perlin in the 10 middle rows and can do 4 single rows on the sides although I reserve one side for a single row of cucumbers.

Let me back up a bit and explain those 10 middle rows supported by the 5 perlins. I plant both rows going to one perlin only 24" apart and then space 30" bvetween the next set of rows. Then I lay boards (over plastic cover) as a walkway between each 30" spacing. Since the stringlines between the 24" rows taper inward I have little trouble navigating between double rows to care for the plants. I typicall get 500 tomatoe plants and 1 row of cucumbers in my 30"GH using that system.

You'll need to buy tomato clips for ease of attaching the plants to the suspended stringlines. I've used black and a clear plastic type and prefer the black ones. The clip simply pinches onto the stringling and encircles the tomato stem just below a branch. Plan to spend a hour per week on the suckering/clipping routine. I try to remove every sucker with the closer plant spacing I use. I remove, sterilize and reuse my clips to save the penny that each one costs. I find that most plants have between 20 and 30 clips each at the end of the year. I purchase the larger boxes of 7500 which you will be best to do to.

To start with I recommend that you determine where you want your rows. that will depend on the criteria discussed earlier. Then you can suspend "S" shaped hangers from the perlins; hangers can be bought in lengths between 12" and 36". I find that the bottom of the hanger works best if it is ~8' off the ground. Plants growing beyond that height need to be lowered. From the hanger I have the twine wrap with ~13-14' of twine on it. Have enough twine hanging to just reach the ground. Now you can lay driptape , plastic and (I put boards down to avoid compressing the soil).

Next the plants get planted thru the plastic being carefull to stay a few inches away from the driptape( I use a stringline over the plastic as a guide). You can immediadety apply the first few clips if your plants are lanky. You will want to make the string straight but not so tight that it will lift you new seeding out of the ground. I typically lineup the taught stringline opposite the first branch and then clip onto the line and angle the clip loop downward and snap closed just below that first branch junction. After each row is in turn on the driptape water and give them a thorough wetting.

I have used a few clips in training cucumbers but I use 24" tomato cages to start cucs climbing and then suspend a roll of plastic netting fron a perlin and let the cucs climb upward onto that. They will attach themselves with their tendrils with very limited training.

I've been fairly wordy in my description. You judge for yourself what methods will work for you. I don't know what you have for a materials supplier but in PA, Nolt's Produse and Irrigation Supply or Reiff's have all the supplies that I mentioned, including crossconnecters if you wish to attach additional perlins (1" swagged pipe from Lowes or Home Depot works well).