As you have probably read in our mag and many others, the 8.5 corporate rearend is very strong and many a drag car has run in the 8s with one that's been properly prepared. Yes, you will need C-clip eliminators and aftermarket axles. We would also add an aluminum rear cover with load pins to reinforce the rear axle main caps. To finish off the hop-up, we would install a tubular support that connects the two axle tubes with a brace across the back of the housing. This prevents the axle tubes from tweaking forward and putting undue stress on the axles. Your Richmond gears and Auburn posi should keep up with the rest of the package. You can pick up all the above-listed components from Strange Engineering, which offers a complete line of parts for the 8.5. And when you want to dump the posi, Strange has both aluminum and steel spools to replace it. Strange's Pro Race Hy-Tuf axles are sold under PN A1000, the C-clip eliminator kit under PN A1030, the Aluminum Rear Cover under PN R5201, and call for the rear brace kit. This will give you a very strong 10-bolt diff. If you are going to foot-brake the car and stay away from transmission brakes, this should give you years of trouble-free service. Give Strange a call at 847.663.1710 or check online at strangeengineering.com.
As for your 9-inch rearend, yes, the Cougar rearend is almost a direct bolt-in. The width is perfect, but we would cut off the spring perches and set it up with 1.5 to 2 degrees of positive downward pinion angle With the Cougar rearend you end up with a stock Ford rear-end. These axles are equipped with 28-spline axles. You would need all the components that you have above and more. It's a coin toss which one would cost more. Yes, ultimately, the 9-inch would be a stronger rearend. However, to build it bulletproof, you would still need an aftermarket third member, gears, spool, axles, and back brace. Unless you're going to put a much more powerful engine or a transbrake (which would kill your TH350 trans) I'd stick with your 8.5. Good luck!
If you have technical questions for Kevin McClelland, send him an e-mail at chevyhi@primedia.com.