Is the computer still in use on this vehicle? if so what knock sensor are you using? if your using the one on the 305 this won`t cut it, knock sensors work off frequenceys, and different bore sizes create different frequencey`s and the 400`s bore is a he** of a lot bigger than a 305`s, 4.125 compared to 3.736. The knock sensors job is to pick up detonation and control the timing down to keep detonation away, but it can`t cure a problem when the grade of fuel isn`t enough for the compression ratio given, it`ll just keep retarding the spark timing until the engine won`t run. second, if your using TBI it will need bigger injectors, but on both counts here, neither a knock sensor or injectors were ever made for a 400 small block.
If the computer is still in use a custom chip will also have to be made, were talking 95 more cubic inches.
Lastly, 305 heads on a 400 was not a good idea, this put the compression ratio in the high 10:1 range, the noise you hear is detonation, which will quickly destroy any engine. the compression ratio is too high for pump gas, and the tbi can`t provide it with enough fuel being it was intended for use on a engine with 95 less cubic inches. you`ve mentioned you`ve reset the timing, with the computer in use are you disconnecting the plug at the distributor while your doing so then disconnecting the battery afterwards so the computer will dump the previous setting and take set of the new one? the engine will want less timing due to the higher compression ratio. The engine is likely running so lean it will likely burn a hole through the piston or cook some valves.