That's a result
A friend of mine has a set of Harrington bumpers on his 250 California replica and they are very impressive.
I've measured the distance between the bolt centres on my bumpers (see picture), it's 96mm, so add to that however much you think you need for side to side adjustment (I would guess about 20mm to each side) to give you the length of the top of the T piece.
The length of the tube part depends very much on where your frame is positioned, a bit of extra length will allow it to slide forwards and backwards until you get the gap between the bumper and body correct.
As for what the correct gap should be I have no idea so I will be setting mine to what I think looks best.
I will be fitting mine close to the bodywork as they are original Dino bumpers and, quite frankly, are so thin that they are for decoration only, in the event of a hit they will just deform so I can't see them doing much damage to the bodywork.
From what I've seen the Harrington bumpers are a much more substantial item so I would move them away from the bodywork a bit to avoid bodywork damage in the event of a crash.
I've attached a couple of pictures of the fronts of Dinos which give a bit of a view of the gap between the bumper and the bodywork.
If you want to look at some more pictures for reference I would recommend Fantasy junction's website. They are a dealer of collector cars in the USA and they take loads of closeup pictures of their cars for sale
https://www.fantasyjunction.com/sold/search?q=dino+246