The 1.8. These FU 750 subs, as well as the sa-15 which is generally inferior, can take 1.4kw CLAMPED, as I explained to you before in a post several pages back. I also explained how with impedance rise you'll usually be getting half power of what your amp is rated for anyway. So you could technically be okay even with a 2.8kw amp on certain frequencies for short listening periods. Definitely get the 1.8kw. Plus, extra headroom means you can play at your preferred listening level with less or no clipping compared to an amp that just barely gets you there, protecting your subs in the long run. BTW, 2 ohm dvc means you can wire them in parallel to 1 ohm nominal or series to 4 ohm nominal. Wire to 1 ohm, your amp will only be seeing like 2 ohms when playing music and the sub will see like 1kw. If anything you'd be underpowering it haha. It's better than a 1.3kw amp though and since you have stock alternator you probably should stay under 2kw. The 1.8k is def what you should get
Unless you can build the box, get cscstang to do it. Very good guy. Make sure to request a vertical dowel and double baffle
TBH there is not going to be much of a difference between the 1.3 and 1.8 but the 1.8 would be better