found a chart to help find suitable spring sizes: Ironman 4x4 Suspension - Leaf Spring Specifications
all the sizes include 40-50mm lifts, but at least its ballpark figures for comparison.
NISS003B is MQ front and NISS005B is MK front leafs. They are the same sizes though, 560f 550r 185/170c. 6 leaves.
NISS004B is common rear, 604f 838r 250/235c. 8 leaves.
FYI, my codes mean f for front half the spring, r for rear half the spring, c is what they call camber (basically they spring height).
I have listed 2 c values because they have 2 available, one drivers and passenger side, which is interesting to note the 15mm difference.
hilux from 97 to 05 are all listed with the same suspension parts. the rear leafs are models TOY027 A or B or C (heavier load carrying with more leaves). They are the same sizes though, 560f 740r 200c. The A model has dual stage leaves, 6+2.
so those hilux leaves are nearly 200mm longer to the rear and the same on the front, which is what Screwy was saying (the diff wont move if the front eye is located same position). They do have more camber than the stock MQ leaves though, which means they arent as flat are are more of a U. But the extra length should make them flex better.
suprisingly, the MQ rear leaf is one of the longest they make. only ones longer are TOY047, which also have a little more camber and are dual stage. these are the rear springs of 79 series.
edit- here is a link of some springs on american 4wds, for reference: http://www.pirate4x4.com/forum/showpost.php?p=10232264&postcount=2
i presume they are stock height lengths though. Interesting to note that the MQ rear leafs on ironman website measure ~57 inches long, which is comparable to one of the longest on that chart.