Mustang 2 drop spindles are the cheapest and most durable parts that you can buy for the front end. But, they're heavy. I really recommend you research suspension design and geometry before you jump into something like this.
I spent many months designing mine and now it sits in the back on my mind waiting to be unleashed once I'm an adult and have a real income to afford such a project.
Here are some good sites:
Home | Reverse Trike
LocostUSA.com • Index page
Those 2 should get your mind going. Please don't buy any internet plans. They're junk.
Consult with people who have been there done that. Tell them your goals and your price and they'll tell you what can be done.
Here is a basic sketch-up model of what I am going to do with my trike. I am estimating 150 MPG at 60 mph on straight gasoline. Less than 500 pounds. Super low center of gravity for great handling. Good safety. Highly aerodynamic.
Keep the questions coming. If you're serious about this, you can end up with an awesome ride.