Suspension travel and geometry are terms that are bounced around a lot without always being defined properly, especially in the context of street/hot rods.
Most cars (not rods) have approximately 5"-6" of useable suspension travel, with about 2/3 of it compression, or down travel. This is why most cars sit as high as they do. Most rods are lowered substantially from stock ride height.
When lowering a vehicle, the suspension travel is also reduced. Most rods only have about 4" of travel max, again with 2/3 of this in downward travel. With this reduced amount of travel, geometry changes become less severe, so some of the compromises in geometry can be reduced also.
For example,the length of a panard rod on a front axle becomes less critical when the travel is only about 2" above or below center. The difference in length change between a 15" bar and a 30" bar in this amount of travel is un-noticable under normal driving conditions.
A little common sense in these situations will make building a rod much easier.
Another issue is ground clearance. As stated above, most rods are lowered substantially from stock. If your rod only has 4" of ground clearance at the grille, 4"+ of downward suspension travel is unusable, therefore there is no need to try to accomplish it.
Besides, on most roads that are paved and reasonably maintained, 2"-3" of compression travel in your suspension is plenty. We have run rods with less than 2" of compression travel all over this country without incident, and with over 100K miles on them, they ride quite well also.