Don't worry about the length, unless you have taken off more than 1/8". There is a lot of room for error. The important things are that the cap will screw down over the tip, and that all of the parts will fit inside of the pen. The most critical length is with the roller ball, and even here there is almost 1/4" of compression in the little spring behing the roller cartridge. The Schnidt reservoir is longer than the one that comes in the kit, and this could be a problem. However, that extra length is a problem when the tubes are as they should be, so don't worry if it doesnt fit - it doesn't fit when the tubes are full length either.
Make the pen, and don't worry about it, unless you have taken more than 1/8" off their pength. Anything less than that and they should still work.
The ElGrande is an excellent pen. The medium nib is more like a "broad" but I think it is the smoothest writing of all the fountain pen kits.
