I have been reading alot of books on building small block chevy engines. It seems like the use of 1.6 rockers is pretty standard on an engine build for street performance. My question is how do you determine if you can use them, or if you should?

I have a 355, a set of double hump heads, an XE268h cam, and a holley 300-36 intake. Do the heads need longer studs? Seems like the arc would be different using the 1.6 compared to the 1.5. Sorry, I've read up alot on this, and never really seen it explained before.

They are in most cases a bolt in. The question on when you use them is simple, if it makes more power. On the dyno we test with 1.5's 1.6's and sometimes 1.7's, switch intakes, exhausts on mutiple pulls. Sometimes an engine responds and sometimes it doesn't. No matter what you read, there is no hard fast rule. Only testing will tell you for sure. Power gains are anywhere from -5HP (yes a loss) to +30HP. Rockers are a tuning tool just like carb spacers, headers, and carb styles.

They are in most cases a bolt in. The question on when you use them is simple, if it makes more power. On the dyno we test with 1.5's 1.6's and sometimes 1.7's, switch intakes, exhausts on mutiple pulls. Sometimes an engine responds and sometimes it doesn't. No matter what you read, there is no hard fast rule. Only testing will tell you for sure. Power gains are anywhere from -5HP (yes a loss) to +30HP. Rockers are a tuning tool just like carb spacers, headers, and carb styles.
Ok, then probably my best bet is to just build the motor, drive it for a while and get a feel for it, and throw some 1.6's on there and see if it improves? I don't see myself springing for a dyno somehow, $500 a pop is a bit of an expense.

What is the difference between the two? Is it the distance between the centerline of the tip, to the centerline of the rocker?

The pushrod cup location.

That cam has about as much lift as the heads will handle unless they've had some serious rework, I doubt the higher ratio rockers would show any more power.

Also make sure you check your piston to valve clearances and make sure your valve springs can handle the extra lift (usually about .030" more) the 1.6 rockers will give you. So you don't get coil bind.

Valve train geometry should be checked too. You might have use different length push rods.

If I was building from scratch Id buy 1.6 and the correct length pushrods and go from there, Im not sure why but they unstalled 1.5/1.6 on my motor. I havent read about that stuff in years.