The leak down test will just tell you if it is valves or not. It cannot tell you what aspect of the valves has the problem. I was talking about where you would have to lap the valves. There are only a few things that will keep a valve from seating correctly. Adjustment (usually for older cars from push rods), springs, bent, or burnt or like what you theorize is wrong. A stuck valve will show up on a compression gauge. Actually several different cyl conditions will show up on a compression gauge and they are easy to get so that would be probably the easiest and fairly fast. Grab a remote start switch while you are at it so you dont have to crank the motor from the inside.
Yea if its not broke dont fix it. But 250k miles is a lot of wear so chances are sometime in the next 5 years or so you would have had to work it anyway. Its better when you plan out this kinda stuff rather than need a tow on Monday, rides to work for two weeks and cash for a motor. BTW, crate motors are pretty cheap and easy. I got a long block a few years ago without bells and whistles for around 1k if I remember right. My junkyard motors always costed a little over 200 bucks but man, that brand new motor (and fresh trans while your in there) feels like a new car. You'll get there and your saving boatloads of money as well as learning a ton of info.
