That car sounds a bit small for towing a 5x8 - I could be wrong but I'm just barely squeeking into the weight requirement with my ford focus.
My owner's manual says that the car can tow 1,000 lbs - sure, no problem for a teardrop but then it says that the tongue weight cannot exceed 100lbs

Now, that is a bit tricky.
I'm certainly no expert at this but here's basically what I know - It is possible to affect the tongue weight by moving the axel. However, moving the axel also affects the towing performance. If the trailer waggles all over the road (and tosses your little car around) that would not be cool.
I just did a quick google search and came up with a few sites that said that a 2001 honda accord has a towing capacity of 1000 lbs and max 100lb tongue weight. (check for accuracy because those figures did not come from Honda)
It seems to me that a completed 5x8 trailer would weigh about 25% more than an identically completed 4x8 trailer. Sure, that is a rough estimate but I don't think that it will be too far off.
I guess the question now is if it would be possible to build a 5x8 trailer that would come in under your weight rating.
The 4x8 Kuffle Creek Cubby is about 900lbs with a 180lb tongue weight. I modified those plans a bit and left my axel in the "stock" positition to compensate for the tongue weight.
I like the HF trailer and I didn't modify mine at all. The modifications in the Kuffel Creek plans are to move the axel back - that does somewhat improve towning performance but I think that it is mainly for cosmetic purposes. My tear tows beautifully with no wobble or anything. Once the axel is moved back, you would have to reinforce the sides because they're not meant to support weight on their own, they're meant to have the axel under them. If you don't move the axel, you don't have to modify the sides either.
I'm happy with my HF 4x8 - I feel like it is strong and safe for highway use. We travel just about every weekend during the summer. Some trips are several hours away. We've only been using the trailer for a few weeks now I know that there are several others on this board who've had their HF trailer for many many years with no problems. I really think that the value for the price is fantastic.
It has been an amazing project - I'm not even close to being done. I'm looking forward to watching your project progress too. Please be sure to post pictures.
www.myphotoalbum.com offers free online albums.
Lindsey