I went with a five lug kit on my 54 for a couple reasons. The most obvious is having all the components, and especially the front wheel bearings, available over-the-counter at your local parts supplier.
The 6 lug kits use the stock hub, which more than likely is fitted with the ball-type bearings. These are fine for low speed, bias ply tires on stock width rims but if and when you add wider rims, radial tires and travel at higher speeds they can present problems. The replacement roller bearings are special order, expensive, and available only from mail-order vendors. I'd hate to be sitting beside the road with a bad bearing waiting for the UPS truck to show up in 3-5 business days.
The 5 lug kits use a fixed bearing spacer that stays with your spindle, a hub/rotor/bearing combo that you can purchase at most any parts supplier. The 5 lug conversion also pushes your rim center out an additional 1" which helps wider rims/fat tires clear the stock or upgraded tie rod ends. I run 235/70/15's on 15X7 rims w/4" backspacing on the front, with plenty of room to spare.
The downside of the 5 lug kits is that if you are planning on keeping a stock rim setup with the original hub caps you might be looking at some custom machine work to retain the "stock" appearance.
As several pointed out, if your plans are to drive your truck on the modern highway and open up your drivetrain there are plenty of diffs with 5 lug 4-3/4" bolt patterns that give you taller R&P ratios, posi and discs. The 2nd and 3rd generation Camaro's (5 lug-62" wide) are my preferred donors, are relatively inexpensive, available, and you can find low mileage units with your preferred ratio.
I hope this helps you with your decision.
Dave
