I gather from your reference to A5.18 that you are performing an all weld-metal tensile test and transverse weld-metal impact test specimens. These test are usually performed by the electrode manufacturer to prove that the product meets the AWS specification. IF you are having to prove out the filler metal you are using then the base plate material should have a 22.5 degree bevel with a 1/2" root opening. The angularity or flatness of the test plate is important for machining out the impact test specimens. This test is only for the purposes of testing the filler metal and as such you need to insure that the plate will pass all of the mechanical test required. I have used a fixture to hold the plate in position and to help prevent warpage. I have also offset the plates by a small amount to also offset warpage. As to the preheat and interpass temperature. It is extremely important that the maximum interpass temperature not be exceeded. DO NOT speed the cooling of the plate when it gets too hot. Rapid cooling will cause larger and more course grain size and will make it very difficult to pass the impact tests.
Another thing to consider is to make a larger test plate than required by the specifications to ensure that the plate doesn't get too hot to fast. This will also speed the completion of test. After reviewing the specifications, I would like to make note that the temperatures you mention are in Centigrade and not Fahrenheit and yes you do need to stop welding and allow the test plate to cool in still air. The point of measurement in at half the length of the test plate 1 inch from the longitudinal centerline.