Well here goes.
3500 feet is plenty long for a normal T6. I landed one on 1800 feet runway when I was learning with John Hess in Texas. Grass, if smooth makes the rollout easier, but hard surface is fine too.
I never heard of a T6 whose flaps were modified like that. One of the things that help a T6 land is the effective flaps. I do>n't know how much you lose with the holes in them. If it was my plane I'd probably try to get some stock flaps and have them installed. There is a place in Dallas, Tx that may have T6 flaps, can't recall the name now.
I assume that you have a T6 pilot manual. If not get one. Here in the U S it would be illegal to fly the plane if you did not have the manual or pilot notes in the plane.
Look up the section on stall speeds. Pick the one that is for your planes weight as you are flying it. The lighter weight gives a lower stall speed. Pick the one for power off, gear and full flaps down. It will likely be around 60 mph, within about 5 mph.
Then fly the plane and have the pilot do the stall, and see what you indicated speed is . This way you can know whether the modification to the flaps really affects stall speed. Let's say it stalls at 62 mph. multiply this by 130% or by 1.3 and you have the speed you want for short final approach. You can be faster on downwind, but coming to the end of the runway, 1.3 is best. That would be 80 mph, with gear and full flaps down, and power off by the time you are at the flare. DON'T BE WAY TOO FAST, and use full flaps. At 10 feet above the ground slowly level off and as the plane gets to 1 foot above the runway you should be nose up slightly, (like the plane sits on the ground) and slowing near stall speed and just about hovering, very little descent left. I prefer to touch on al 3 wheels at the same time, (3 point) but it can be on the mains first. DON'T try to force the plane down on the main wheels if still too fast, don't push the nose down below level. You want to have almost all the vertical descent speed used up and most of the extra airspeed used up when the wheels touch. A T6 seems to be able to wheel land about as well a 3 point if done correctly. I prefer 3 point , that is how the RAF and our Army airforce taught to land the fighters, but it would be good to know both.
When I last flew a T6 it was actually a Harvard. I had never flown a Harvard, the owner just said have a go in it. I was flying a fighter of his, so I guess he had some faith in me, and the $ amount at risk was less than $200k as opposed to $2million for the fighter. I used 70 knots at short final ( like 80 mph), full flaps and power off, for a 3 point landing, and turned off near the midpoint of a 2700 foot grass runway.
Make certain to have the plane lined up straight with the runway. If there is no crosswind you should not have that much problem with the tailwheel trying to move sideways. If there is a strong crosswind, don't fly that day. Use the rudders to keep the nose straight on rollout. If you really get off line you may need brake ONLY AFTER THE RUDDER. Try not to get in that situation.
It can be done, that you learn from scratch in a T-6. I know a man who did in the military. But it is much better to get a lot of tailwheel training in something else first. A Tiger Moth or Chipmunk or J3 Cub, are good.Get used to not being able to see straight out the front on the ground, and how the steering and rudders work.
Do a lot of taxi practice before you fly, especially in the T6. I prefer to start in the back seat of the T6 so you get used to the restricted forward visibility.
It is a great airplane and a great warbird trainer. It is not a Cessna.
A very good book to have is called TAIMING THE TAILDRAGGER. And you need a good teacher.