The engine is being built to put out about 350 hp but i am doing some other modifications that will put it close to 400 hp. How much can the transmission take?! Its a 79 L-82 with a manual transmission.
If your tranny is original, it's either the M-21 close ratio or the MM4. The M-21 had been around since 65 or 66 (failing memory) and they can handle whatever you throw at them. The same is true for the MM4, the M-21 just has a lot more history.
M21 was really only an RPO and not a specific type of transmisison.
From MY66 to 73 M21 was a Muncie, close-ratio, standard-duty transmission. From MY73-79 M21 was a "Super" T10.
While the input torque rating of the Super T10 as used by GM in the 70s is unknown, Richmond gear, which now owns the Super T10 design, lists its input torque rating at 375 lbs/ft. for a trans. with a 2.43 low gear ratio.
M21s, regardless of manufacturer, are not "good" for up to 500hp as was said elsewhere in this thread. I'd say the transmission would have marginal durability that that power level and unacceptable durability if the car was drag raced freqently.
My guess is, as long as a Super 10, in good condition, is not drag raced frequently with sticky tires, 425 lbs/ft input torque will not greatly compromise reliability.