What kind of money are you willing to spend? That is key. Any engine combo that has been mentioned should work just fine for 8K. Heck, my dad's 1/2t burb with a Vortec 350 handles 6.5K respectively.
I'll chime in on the Dodges for you. Avoid any small block Dodge pickup. The 318 is one of the best engines ever made but it's an engine for gas mileage and grocery getting, not towing. The 360 is POS, sucks gas and it's power just sucks. The Hemi is basically a joke in anything larger than a 1/2t, they should have kept it in their throwback "sport" trucks that never pull anything.
The later model Dodges with the V10 are GOOD pullers. They are huge, nearly 500 ci IIRC and make some good power. Before the 24v Cummins, the stock V10 power numbers were impressive even compared to the Cummins, but thats on paper. You will find the 47RH or 47RE auto trans behind the V10 in the mid 90's to very early 2000's. I've heard the autos failing behind the V10 is less common than one behind the Cummins. I wouldn't be scared of an auto V10 at all, I have a 47RE equipped 24v Cummins truck and have had good luck. I will venture to say the 47RE tranny is as stout as the 4L80E. The difference is the 47RE is in a tougher application. You can also get the NV4500 manual behind the V10 also, there are a few guys with either the auto or manual over at towrig.com and they love them.
Also, the later 94-02 Dodges are nice trucks inside and out, on the later versions. Nice trucks that are solid for a fair price vs. an HD chevy or GMC. The best flip out towing mirrors in the biz too. The V10 has a cool, unique sound like nothing else too.
Don't know much about the Fords.
Our '98 C30 Dually with a vortec 454/NV4500 is a decent puller. It actually pulls some decent mileage, like 14 on the highway and 10 in town. I don't think it pulls like my neighbor's V10 Dodge though, but in all fairness it's a larger engine than the 454.
If you can go diesel. It's the way to go when you are going to drive a lot and tow a lot.