The 45 was a strong box but a technical dead end, it added a 4th speed but the first gear was ultra low and only reached to about 12 mph while the 2nd a 3rd gear ratios weren't as well suited to 1500 to 2000 cc cars as the old 35. Only problem I ever encountered with the 45 was a broken kickdown cable -- slightly different from the 45 the cable quadrant was inside the transmision and changing it meant dropping the gearbox sump.
GM slush boxes were always good as was the Chrysler Torqflite