I wouldn't even try to modify the Arca to have base tilts when they aren't engineered in. The 4x5s with base tilt (the B) and combined base and axis tilts (C) are pretty common on eBay and ultimately you could buy a couple of cameras to assemble your ultimate kit, and selling off the extras usually at a slight profit.
I like base tilts better myself, especially for landscape work where it is a simple matter to focus on the horizon and tilt the back towards you until the foreground comes into focus. But for most other applications axis tilts get the job done, and if your compositions are centered, as most portraits and studio work tends to be, the axis tilts work to your advantage. But the choice is hardly a deal killer because lots of great photos have been made with either type of tilt, regardless of subject matter.
The newer (>1980s) Arcas are CNC machined and so much more refined, smoother, and tighter. However, the older models have no real shortcomings other than age and more plastic parts - knobs may crack or levels my dry out - none of which is a big deal.
The only gearing is the focusing track, which works fine and is easy to clean. The base rail clamp fits into any Arca-Swiss clamping track, and the 8x10 is easily handled by my light 3 series Gitzo and Arca-Swiss B1 ballhead. That was my issue with the Fatif - it was simply too heavy and required that I purchase another heavy duty tripod. The Arca 8x10 is light enough to work with a lighter tripod. It locks down tight and has a bail for the back, which makes inserting holders a much easier situation (you're less apt to shake or shift the camera with a smooth holder insertion).
Of course wind will still catch the bellows and this is where sheer mass (a Fatif or Sinar P with a No.5 Gitzo) will be steadier no matter what.
Any of the older Arcas -- you need to inspect the foam seals on the bellows and light traps, as these usually are dry and crumbling. The lock downs can be adjusted but not the detents. A Sinar Norma, which only is geared on the standards, not the entire rail, is better in this reagards because you can rebuild everything and reset the detents to acheive perfect alignment. But it is a bulkier camera (designed by the same guy as the Arca!) but I think ranks slightly higher than the older Arca.
Last year I had a 4x5 Arca F and it was the finest camera I've ever had, and I've had most of them. If it weren't for the expense and lousy customer service I'd be even more pro-Arca, but the company has it's head up it's French Fried buttocks.
Bob Watkins of Precision Camera Works (Dead Link Removed) is the authorized Arca service in North America and he is a great guy. He used to work for Sinar and knows his stuff cold. His website has a lot of Arca info (some of which I wrote!)