Will timing advance effect engine temp.

Initial timing is a function of the duration of the cam you are using. More duration requires more timing to ensure a complete burn of the mixture at idle and lower rpms. The total mechaincal is a function of the design of the heads and how efficeint they are. Typical numbers for open chamber LA heads is 34-36 degrees. The magnum head is a much better and more effecient design and peak power is obtained with less, in the 32-34 range.

You determine the initial timing the engine wants by increasing the timing in small steps while matining the idle rpm to find the peak manifold vacuum. If your 340 has a stock or close to stock cam in it then ideal initial timing should not vary much from the a few degrees from the factory recomended number.

Total mechanical can only be accurately determined on the dyno.