If by "easy" or "bolt-on" you mean "without modification", then you are stuck with the A4LD. I'm not an expert on such swaps, but from what I've gathered, there are a few available options:
1) If you don't care about 4th gear or torque convert lockup clutches, you can put a C5 in there. It will likely be a different length than the A4LD, so at a minimum you'll have to lengthen/shorten driveshafts. This gets done occasionally, and should be fairly well documented on this or other forums.
2) If you're interested in a manual transmission, the M5OD was used in Explorers and is the same length as the A4LD. Should fit in fairly easily once you figure out clutch components (clutch pedal etc.)
3) If it's "easy" to pay for the adaptor, you can put a 700R4 behind the 4.0. This swap is documented in a thread here somewhere.
4) I've been watching the 5R55S thread. The 5R55E is reportedly almost the same transmission as an A4LD, except that it's electronically controlled (and strong enough to hold up to a stock SOHC). Gut feel is that, once they figure out the transmission controller, it won't be difficult to put a 5R55E into a 1st gen. The main difficulty might be paying for the controller.
5) Along these same lines, according to the A4LD rebuild diary and the Frankentranny write-up, you can reportedly use many of the 5R55E internals in rebuilding an A4LD to make it a stronger tranny.
Others may have other suggestions, but that's what I have to offer.
