I bought a new Duralast Gold battery (H8-DLG), which fits in the well but the cables are not long enough to reach the posts. The terminals would also have to be changed to the right-angle type seen in the image for them to fit.

Are there any reasons not to use slightly longer cables (of the same gauge) or the right-angle terminals?

It would obviously be easier to return the battery, but I'm not sure I can and my original cables and terminals are not in great condition. Is replacing the two positive cables difficult?

I have a 2000 Discovery II with 96K

 

Not sure why you would need right angle terminals. H8 is a top post battery which is correct for the Disco.

My SuperStart (Oreilly) battery came with a spacer for the bottom of the battery. Without this the battery sits too low for the cables to connect very well. If you don't have this spacer, a piece of wood or something about 1in thick would be fine.

All that said, if your cables are in rough shape, they should be replaced. It's not a hard job, if you can change the oil you should be fine.

 

I recommend returning the battery for one that fits and takes up as much space as possible. I have the NAPA 8424 https://www.napaonline.com/p/NBD8424

If you want to take it a step further, here's what I did
1. Replace the negative earth cable w/ 0 gauge welding cable + eyelets
2. Cut off the positive lugs and replaced with eyelets
3. Installed mil spec terminals

 

Returning/swapping for a different battery is fine but when I had the same problem I used a short section of 2x6 under the battery as a spacer.

 

I bought an Optima red top which turned out to be much smaller in dimensions than the old battery I was replacing, luckily Optimas come with a top cover that can be flipped onto the bottom like a booster seat to push the posts up higher. I still had a bit of room along the sides so I took some big chunks of rubber cut from some old F150 radiator hoses and wedged them on both sides of the battery. Nice and snug now.

 

Jeff, your engine bay is cleaner than the plate I just used for dinner. Bravo!!!

 

I'm going this route when I do my Big 4 cable conversions on my 2001 Disco. I am so used to the Mil Spec battery connectors since being prior military maintenance mechanic that this is the best way to go.