I defined them as:
in #75
source: 1912.0; 6.0; 1796.0
dest.: 347.12; 16.86; 43.96
in #76
source: 1883.0; 0.40; 148.0
dest.: 406.10; 16.90; 45.30
Note that released US 75/76 svmap don't contain the def. of the portal to map #81; but the 75/76 wld files contain:
#75.wld
source (1910.068, 4.427, 1793.428) - (1913.047, 9.877, 1796.204)
dest.: #81 at (346.48, 16.60, 45.07)
#76.wld
source: (1881.999, 0.516, 145.70) - (1884.585, 4.216, 148.840)
dest.: #81 at (405.62, 16.60, 44.17)
as indicated by nubness, a few tries in game with the bmove command lets you figure out these coords yourself and without any difficulties.
however I do relist them there to double-confirm them after

where some ""invalid coordinates"" are discuted without knowing if the toon location is correct but the arrow displayed on map incorrect or opposite or both.