I've got a 70 Mach 1 blow some smoke on start up and when I rev - hoping it's just valve stem seals - sat for a long time. In any case plan on doing a compression test this weekend and then a leakdown if warranted.

Found a lot ofgood information on compression tests but not much on leakdowns. Can anybody point me to Leadkdown test for Dummies post or site? thanks

 

I've always just done the regular compression test first, then I squirt a small abount of thin oil (like sewing machine oil) into the cylinder throught the spark plug hole, and redo the compression test. Comparing the two results for each cylinder will tell you if its your rings or valve seats. Ie if the wet test is higher, you have ring problems. if it mains the same, you have valve seat problems.
This does not directly tell you anything about you valve seals. It tells you the condition of your compression rings, and this is indicative of the condition of the oil scraping rings. If the rings look as though they are ok form doing this test, and you are blowing smoke, this then indicates that you may have valve seal issues.
BTW smoke on startup is usually indicitive of valve seal wear, rather that rings anyway.