Please pardon another newbie question.

I've selected a 20" barrel with a rifle-length gas port position (SI Defense .308 SASS contour).

Do I need to select a particular gas block attachment mechanism?  Or ... is it all up to me whether I get one that clamps on, attaches by set screw, etc.?  I see 3-4 different attachment mechanisms listed.  So what I'm asking is whether a particular barrel will require a particular attachment.

2nd, will a rifle-length free float tube cover the the gas block (set at rifle-length gas port position) or would a railed gas block be a reasonable option?

Thanks!

It's up to you what type of attachment you want to use. Using pins will require some work, whereas a clamp on or set-screw type will do what the name implies.

A rifle length (12") free float will not cover your gas block. You would need an extended one, 13.2" and 15" are the ones I've seen.

Thanks, attworth!

In that case ... using this tube (probably) ... http://www.midwayusa.com/product/737264/dpms-free-float-tube-handguard-lr-308-rifle-length-aluminum-matte
I won't rule out a railed gas block.

It all depends on what you think you'll want up there. A sight or something would be the only thing I could think you'd want on your block if you're free-floating anyway. A bipod or sling attachment kind of negates the free floating benefit.

Be sure you post pictures when you're done!

If you think you need a front sight then a railed gas block would give you a mounting point in font of that rifle length handguard.
Make sure that you get the correct height sight.
Also, you may get a better gas seal if you go with a steel gas block as opposed to Alu.

If you want a longer-than-rifle handguard, make sure your gas block is low enough to let it pass over.