8.5s sound pretty fast for a FMS D/S, so maybe it'll hold...
I have one, I just hate to get everything together and end up busting it. If need be, I'd almost rather sell it and get a good one unless this one will hold up to what I plan on putting it through.....
Dude! Your car/build is way to nice/powerful to even be thinking of using that driveshaft. You would be better off taking your chances with the stocker! If you do some research on them you will hear about some (well, more than some) people having success and going really fast with the fms driveshaft. Renegade racers using them and people going 8's for years with them. The problem is that it is totally hit or miss and there are dozens of 400hp cars that have mangled them. Allow me to possibly save you some time and put up some "food for thought" I have saved from back when I was in the market and researching them (sorry, no links):
"well i guess im the only one that runs 9.70s at 3150lbs, and took the ears clean off the FMS aluminum shaft...funny thing is the car goes faster with a steel shaft that weighs 8 lbs more?"
"I, after having read and seen so many FMS/Ford driveshaft explosions, decided that when it came time to get a driveshaft done for my car with the twins, I wasn't going to spare any expenses. Rather than getting my FMS aluminum driveshaft shortened and rebalanced, I decided to have one remade. Why? Strength!!! The FMS unit comes with .111 wall thickness with 1330 yokes/joints. The 1330 isn't that bad, but .111 wall thickness is TERRIBLE. IMO, any car running 500+rwhp should have a driveshaft with a thicker wall thickness. Because they're so thin, if you ask Ford, they'll tell you they're really only rated to about 450-500 horsepower. Granted, thousands of people have probably used them beyond that, but there's enough breakages that I'd rather get something stronger."
"Well guys in the quest for an 11 second motor ET tonight I just destroyed my brand new two week old Ford Motorsport Aluminum Driveshaft. "
"a friend of mine just this weekend pitched his aluminum one out. it broke one of the front ears off at the u-joint. his car weights 3000 ish, and 60's low 1.40's. jason smith at pa, said anything that 60's good it will pitch it."
"my buddy twisted 2 in a weekend @ little over 800 hp and ran 9.teens @145-147mph"
"I broke one and twisted another and at the time my car was only making 800 RWHP. One of the shops where I took one of the DS to be repaired showed me the thin wall thickness that FMS uses and it was totally not acceptable. I had a custom steel one made and have been happy since."
"my friend has twisted 3-4 aluminum shafts and has gone back to a steel one"
">>choice is either a ford motorsports aluminum one or my stock one.
Keep the stocker. I busted my Vasco input shaft but the driveshaft lives on."
"I have a friend that twisted a fms aluminum drive shaft. Right at the starting line It had been in service for a while. The car normal went mid to low 11`s @3100 with 1.50 60 ft launching @ 5000-5500rpm. After he broke it went to swap meet and bought a stock steel one with no problems breaking."
"I've seen FMS aluminum DS fail on stock Stangs, usually at the u-joint coupling. But I've also seen them run without problems on several cars making 500+rwhp. For me it was Carbon Fiber hands down."
"I was racing at the track last week & destroyed an aluminum driveshaft. when it let go it banged up the undercariage pertty good on my prestine 93 Cobra. More info on why/how it broke: Launching off the 2-step for the first time & on new ET Drags. 1.57 60ft on the second run (dead hook), 1.63 on the pass that the DS broke. "
"I tore up an FMS driveline just like 93PONY did. The two "eyelets" that hold the U-joint "spread" and allowed the U-joint to twist out. It probably would have been a low 1.6, high 1.5 60'... You know, if I would have made it that far. It broke halfway through 1st gear. Banged up my undercarriage pretty good (I have a safety loop, but the driveline broke at the rear)."
So, I guess you gotta ask yourself, do you feel lucky??