The 4.3L V6 uses the same mounting setup as the small block Chevvy V8 - while it's not a "drop-in and go" or "plug-and-pray," it's doable - you just have to adapt the mounting.
It would probably be easier for you to find a donor drivetrain using a 3.1L or 3.4L V6 (best bets would be the S-10/S-15 truck or F-body, but some FWD vehicles got blocks that could go either way - check for a starter mounting pad on the backside of the engine block, as mounted. F-body = Camaro/Firebird.)
The 2.8L was underpowered from the factory (the AMC four makes more power, and more useful power!) and leaks like a seive. I had one in a 1985 Cavalier for two and a half years - never had to change the oil. Just change the filter every two months, and keep the oil topped off. I put a lot of work into trying to seal that damned engine... The 3.1L was a much better engine (had one in a 1992 Corsica) and didn't leak.
The 1984-1986XJ was designed to take either the 2.46L four or the 2.8L V6 - and the 3.1L/3.4L V6 are externally similar to the 2.8L anyhow. So, all the parts are pretty much "off-the-shelf."
You can also go nuts and swap in a V8 (Advance Adapters makes a mounting kit for the Small Block Chevvy,) but the inline would be too much work. It's doable - but the front clip was redesigned to accommodate the inline six for 1987 - firewall massaged, radiator support moved forward, ... It can be done - but it sounds like you're a neophyte yet, and I don't want you to get discouraged by a project that looks impossible at first without having a chance to build up some skill first.
The V6-60 (3.1L/3.4L) would be the easiest swap, followed by the V8/V6-90 (4.3L/SBC). But, there's really nothing wrong with the AMC four, you may as well keep that! The only real issue I have with the V6-60 is that they tend to be "high-strung" engines, meaning that you usually have to wind them up pretty good to start making useful power, while the AMC four gets you moving handily right off idle.
