What are some thoughts/opinions about this set up for elk. My concern is mainly about broadhead choice. My rig will be a 56#@28 longbow, I draw 27".
Arrows are GT 3555 trads 30.5" with 275 gr up front. Total arrow weight is around 560gr and speed is around 160fps. Will a delta 4 blade with its small bleeders work or should I go with a 3-blade VPA or even a  2 blade 135 gr Magnus1?

You're deltas will work just fine.

yep they will work

I would think you would be fine. I will however state I have never shot an elk but seen a couple and the only thing that would impede penetration to me is the shoulder bones.

Sounds like a good set-up with the deltas to me.

I think you're legit with that setup. I think with the bow and arrow weight you could go with any of the well established 2 or 4 blade heads ... with 1 1/8" - 1 1/2" width

Trick,
My favorite broadhead is the Delta 4 blade and I have shot several bulls with them.  Arrow weight approximately the same as yours but with speed in the mid 170's to mid 180's and I have only gotten full penetration on one broadside bull.  I have recently  converted to the 2 blade single bevel broadheads and now will only hunt elk with them. Any sharp broadhead in the boiler room will kill an elk but two holes make the tracking job much easier and in my experience the penetration difference is significant between the 4 blade and the 2 blade singel bevels.

I think Delta 4 blades are fantastic heads. However... for elk at 27" draw I personally would use a 2 blade.They are big animals.

Me, I would opt for the 2 blade.  You start playing with the angles and you have a lot of elk to go through.  If you increase your bow weight, your broadhead options increase as well.

With that shet up I'd go with the Delta No Mercy 4 blade instead

Cant beat a Delta.Get em sharp and they wont let you down.Still one of the toughest heads going in my opinion.

Sorry....on cell....the  reason being draw length makes for 53#s....

Deltas on whitetails all day....but I'd headge on elk a bit

I would also go with a two blade.....more penetration...I like the eclipse with the Polytetrafluoroethylene ( tefelon ) coating.....good luck

Sorry....on cell....the  reason being draw length makes for 53#s....

Deltas on whitetails all day....but I'd headge on elk a bit

I think with that set up, I would go with the Tuff Head on elk.

Well...if not a Delta 4 blade, a Delta 2 blade would definitely be a good choice, if you are concerned about penetration.  A Magnus I or a Abowyer Wapiti are other excellent delta configuration BH's.  Any well made COC traditional 2, 3 or 4 blade head will work fine, all else being the same.  Work on perfect arrow flight and accuracy.

Good Luck

Kris