Unless there's evidence of excessive wear or damage, don't replace the gears. Replacing undamaged gears gets expensive quick. The only exception is 2nd gear, which is commonly upgraded to the Viper and newer F-body design with longer clutch teeth.
Straight cut gears are overkill for a street driven car, plus there will be a noticeable whine. Unless you will be using the car for strictly racing, there's no need for it. The stock gears are easily good for well over 500hp, and can be treated to go even higher (cryo, micronite, REM, etc.). For extreme street use, there are kits available that utilize a lower angle helical cut gears (22 deg., I believe), which are stronger, thicker, and are still pretty quiet, that can take you to the 1000hp mark and beyond. But be prepared to spend $3500 for 1-4 gears alone, parts only.
You can do the work yourself with only a few special tools and it is a very rewarding experience. If you are mechanically inclined and enjoy that kind of work, go for it. You'll save some money and learn a lot on the way.
1995 Firehawk #528 resto-mod underway
Planned 383 rebuild: 195cc competition LT4 AFR heads, custom cam grind (still debating specs), 6" or 5.85" rods, forged internals
Upgraded T56: Viper mainshaft and 2nd gear, steel 3/4 shift fork, Billet keys 1-4, bronze shifter cup, internals Cryo'd and REM'd