The 383 4 barrel engines equipped with the 727 trans had a factory stall speed range of 2350-2650 rpms. Now you are building a 383 with more cam and LESS stall. This will surely hurt your performance. You will certainly need a looser converter, something over 2800 at a minimum. I have a 3000 stall converter in my 440/493. Don't be afraid of using a converter with more stall. A quality unit will still move the car from a stop without excessive slippage.
As far as manifolds, YES, the performance will suffer. The 383 likes to rev and with the hotter cam, you will lose a little on the bottom end while gaining in the upper rpm range. The problem is, just when the engine is starting to make power, the log manifolds become a bottleneck. The Magnum HP manifolds are your best bet. In a totally stock app they are probably worth 10-15 HP over the log type. Add in more cam and rpm and the gain over log manifolds widens even more.