So I'm trying to develop my "legato with stretch technique" and I came up with these basic exercises with 1 and 2, 1 and 3, 1 and 4 fingers stretched (that will be 1-3 fret; 1-4 fret; 1-5 fret respectively) and I started on 60 bpm quarter notes, really slow but I feel some tension on my left hand and arm which I think is normal.

My question is: should I take care of this tension first and start even slower or should I practice at this tempo until it goes away? I'm playing very accurately but I believe the stretch is the cause for so much tension. Or maybe should I change the exercise? Thanks guys.
I think you should maybe start a little higher on the neck and move down to the lower frets as the stretch becomes more comfortable. Always making sure you're as tension free as possible of course.
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Learn to "relax" into stretches. The tighter your hand gets the less you can stretch and maintain effective technique.

That said, the guitar neck is wonderfully designed for stretch exercises - just start at the 12th fret and move down every time you feel comfortable. Stop moving down when you stop feeling comfortable until you feel comfortable.