Are there any way to get (75V 1A) output from 78V input
3V error at 75 is only 4%. Most regulators won't do much better than that. Are you sure you really need your 75V more than 4% accurate?

And how accurate is your 78V? If it drops by a few % it can get below the 75V mark, and your regulator will have to operate in an 'interesting' mode.

The TL783 you found is a variation of the LM317. Most plain LM317's can handle 1.5A, but be sure to provide cooling (even at 3V drop 1A means 3W, which is more than a bare TO220 can handle).

Note that an LM317 can handle only a limited voltage (typically 35 or 40V), but that is the voltage difference between input and output. In normal operation the LM317 never 'sees' the ground voltage, so it happily drops 78 V to 75V (3V is the minimum drop for an LM317). BUT it would have a problem when you ever short the output to ground: then it would suddenly see a 78V drop and it would probably die. (Note that with an 78V drop the power dissipation with a shorted output would be very large.)