Somewhere here on FM is a reply i made with the measurements i don't know if 390s are all the same. This is a '67 dipstick for 390. You should measure the dipstick tube too so you can compare. On mine it's roughly 7 1/4" long from the block to the tip but there is a slight bend to it which might make it a quarter inch longer... And Hush you guys, i know what you're thinking!!
Another way to make sure you have the correct mark is to know what your crankcase holds, if it is supposed to be a 5qt pan, then what you do is put in five quarts and make your mark based on where the oil is on the stick (note: do not start the engine, the filter will take about a half a quart). Then once your mark is made you can start the engine, this should drop your oil from the full line you just made where you can make a second mark to know when you are a half a quart low. Works for me every time i have to do deep sump pans.