The F40 is only good for a light car. Generally, the "rule" for a manual transmission is you don't use it on a heavy car. Semis, trucks, vans etc are different. I guess you could build up a manual to haul a Bonneville with V8 power, but the gears would be loud and the clutch alone would cost over $1000.
The 4T65E-HD was used on another forum'* 536 WHP Grand Prix for two years before finally it broke the hardened input shaft on a track run attempt. They have since rebuilt it and are planning for 900 WHP with Porsche axles, some crazy input shaft, etc. You can do anything if you put your mind up to it. If you want, you can build a 4T65E like the GP guy did and drive it for two years until you tear the input shaft in half. Or start with an even stronger one since I believe today'* input shaft metallurgy has changed for the better.
Anything is possible, as you can see in that it is possible to have a whole thread about people warning you about having more than 300 horsepower at the crank. I might be biased because I badly want to see this done, so please weigh your options and make a sound decision (and then post pictures of the swap!)
94 STEi and 99 SLO