If I remember correctly (I don't have my book in front of me) its 425 and I like no more than a 5% variance high to low. Note that naturals have a significantly higher compression ratio and thus a significantly higher minimum spec.
Note that technique in measurement counts.
As I have said, I'm FAR more interested in a boroscope than a formal compression test. If the "30 second" style that I wrote about passes then you do a boroscope. If that shows good airbox, cylinder, crown and skirt condition the odds of you failing a formal compression are extremely low. You want to do the boroscope anyway, as a compression won't tell you what the piston skirts, crowns and holes look like, nor tell you anything about airbox deposits, and all are worth knowing.
You're going to spend upwards of $600 per engine to have a compression done by anyone who is competent to do so; it requires a full tuneup (including valves) as you must pull all six injectors (three at a time) to do the test, and since you are disturbing both the rack and the rockers the only correct procedure is to do a full tuneup "by the book" when you're done.
Oh, the test is also supposed to be performed "hot". Forget that (unless you want to take a full day - get it good and hot, do the first half, put the three back in you took out, tune the engine, get it hot AGAIN and do the second half), but "warm" is absolutely necessary or you will get bogus results.
After the "cold start" test (that one's free) if I'm still interested I would proceed to a boroscope. If the boroscope doesn't identify a problem or I have a horrid result on the 30-second start test (no boroscope needed in that case!) I would probably attempt to negotiate for rebuilds right up front and failing that, pass on the boat rather than spend $1200 to tell me what I already know. If the cold start and boroscope are both clean then I want to see the boat run; key for me there is that it easily reaches rated WOT RPM (WOT + 50 is what I'm actually looking for if the boat is light, and it probably will be since its being sold) and everything is within spec, including operating temperatures for water and oil along with oil pressure, especially at idle when HOT (right after coming off plane from cruise.) That tells you a fair bit about main bearing condition (20+psi is good, 15 psi ok, 10psi absolute minimum at 550 RPM idle in gear with the engine having just come off plane)
If the boat is overpropped that's a serious black mark on expected engine life, all else being equal. If hot idle oil pressure is low, ditto - you've got a full overhaul including mains in your future.
If the cold start fails the owner knows. Its impossible for him not to. This is the primary reason that 99 times out of 100 the compression test is a waste of money; you're just documenting what the owner already is FULLY aware of every time he pushes the botton in the morning.
If the cold start test fails a boroscope either reveals the cause (in which case you have your negotiating leverage) or you're in for a war with the owner who already knows the engines are below minimums. He's either going to give you a reasonable adjustment or he is not. The $1200 you're going to spend is almost never going to be the critical piece that gets him to say yes and if he says no you're out the $1200 for nothing.
I've seen several 6-71TIBs with compression in the 380-400ish range and while they ran fine once warmed up there was absolutely no doubt of any sort that they were under minimums on a cold start. You could easily use those engines as mosquito control devices in the middle of the Everglades and kill every bug within a mile, and without preheat starting them in temperatures under 60F or so was a SEVERE test of battery condition. Below 40-50F I doubt they'd light without assistance no matter what you did.
On the TIBs the back two holes have a habit of going first. I've never seen a satisfactory explanation as to why, but its a pattern that I've noted over ~20 years of observation. The best explanation I can come up with is that the bypass valve is at the rear of the blower but that's not really a satisfactory explanation as it shouldn't matter - but the fact remains that on a tired 6-71TIB the rear two holes, on tear-down, will almost always be in the worst shape.
