The basic idea of Modification #2 is to disable the onboard oscillator with a jumper and feed in a new signal on the back of the board. There are very few who have performed this mod, but I feel it is more elegant and safer since you don't have to remove the onboard oscillator. This newer, and less evasive method has been performed on IIsi's & Q700's by myself and others, and should work fine on the IIfx, Q900, Q950, and PM8100/110.
The most difficult and risky part of "Mod-1" above is the removal of the oscillator, and this is an alternative procedure that gets around that since the crystal oscillators Apple uses have pin 1 as OE. On a crystal oscillator with pin 1 as OE, if you ground pin 1, you disable the output from pin 3(8), and you can feed a new signal into pin 3(8) without removing the original crystal oscillator.
Several months ago I performed this modification on a Quadra 700 by tacking (soldering) a jumper on the back of the motherboard between pins 1 and 2(7) of the 50MHz oscillator, and ran wires about 8 inches long each from pins 2(7), 3(8), and 4(14) to a 14 pin socket attached to the inside of the Q700 with pins in positions 7, 8, and 14. Into this we placed a 70MHz crystal oscillator and later a 74MHz oscillator. This modification is nice in that it is a bit less risky as far as damage to the motherboard, but you have to be careful to use thin wires in order to make clean solder joints. With this modification you could remove the wires at a later date to return to the original configuration more cleanly.