Mustang Boards - Piggyback style underdrdive pulleys

I am a first-hand user of the piggyback and I eventually ended up purchasing a 1 piece when the funds became available. IMO there are 3 reasons why the one piece design is better.
1) appearance - the piggyback design retains the stock pulley/balancer assembly and just sandwiches one more pulley to the front of the engine so you now have 2 pulleys (one not used) and a balancer that sticks 2 inches further out (and makes getting the fan shroud off really tough without pulling the assembly
2) ease of assembly, getting the piggyback style bolted together straight (remember, this is your balancer assembly) is tougher than you think. I had to do it twice because I didn't clean the inside of the balancer enough and it didn't sandwich together exact. It ended up throwing a "misfire at low RPM" code b/c it was ever so slightly our of balance (same thing can happen if timing adj isn't torqued down evenly)
3) rotating mass / performance: the whole assembly becomes heavier when you add a pulley to the stock assembly and you're moving that mass further from the end of the crank. The 1 piece is much lighter than the entire 2 piece together (less rotating mass) and it's where it should in relation to the crank.

For what it's worth, it definately gets into the revs a little quicker. People seem to have differing opinions on the U/D pulleys, as an old 5.0 person, I just couldn't let go of them for the 4.6. Do what fits best in the budget, for me it was easier to spend $100 a year ago for the March 2 piece and another $75 a year later to get a 1 piece crank pulley.