If you do your homework and a little shopping, the cost difference is only about $80 between the 426 and the 440. The difference breaks down like this: for both kits you must buy the piston and gaskets = same cost. For the 426 kit, you must have the cylinder sleeve bored: average cost = $50. For the 440 kit, you must buy a sleeve ($62.50 from Wiseco) and have it installed into your cylinder. Wiseco will do all the work to your cyliner (remove old sleeve, bore cylinder, install new sleeve) for $50, and I have found a machine shop where I live in Phoenix who will do the same work for $75. This makes the difference in price equal to: (75+62.50) - 50 = $87.50

Like I said, do a little homework and the cost of another 10cc gets reduced.