Crooner Reverb like Sinatra, Martin, Nat King Cole, etc...?

The people you list in my understanding used the room they were in to have reverb. The studios in some cases had their own sound. However, spring and plate reverbs were in use for at least part of the time many of the recordings were made. A very light delay also can be used.
 
In truth a book or article that goes into the techniques and equipment used in those days might be very helpful.  I would keep is as simple as you can.   

Can't speak to all of them, but I can tell you that a big part of the Sinatra sound was the infamous chamber that they used. Nothing sounds like a chamber!

I've only worked in a couple of studios that had chambers, but they were always used!

The rooms in which the recordings were made also add a lot, but I'm pretty sure the part that makes you smile is the chamber.

If you have a Waves collection check out IR-L (or IR-1), they have a bunch of rooms and famous studios you could try as a starting point. 

For chamber reverbs, try the following:
 
1) I assume I have a version of SONAR that includes ReMatrix Solo. If not, never mind.
 
2) Read the info box (Impulse Management) at the end of this Craig Anderton article in Sound On Sound: http://www.soundonsound.com/sos/jun15/articles/sonar-0615.htm
 
3) Download Samplicity's Bricasti M7 impulse library (donation ware; includes a bunch of chamber IRs): http://www.samplicity.com/bricasti-m7-impulse-responses/
 
4) Let us know how this all turns out for you.
 
I got to hear Frank live once in a club where Rush St. ends in State St. (it's Gibson's Steakhouse now) in Chicago. Very cool. But nowhere near the quality of his recordings. He was a master. Always such great arrangements behind him. I've got his early teenage stuff with Tommy Dorsey, his a cappella work recorded during WWII. But his best work was from 1950 to 1970 or so, when he started making movies. I have pretty much everything up to that point...and nothing after.
 
One of the best all time.

Sanderxpander
If you have a Waves collection check out IR-L (or IR-1), they have a bunch of rooms and famous studios you could try as a starting point. 

+1 for Sanderexpanders idea.  This would be a good starting point. 

I know that the OP was asking about SONAR but....
 
Sinatra did quite a few recordings in the United studios in California.... which I think was converted over the years as Ocean Way Studios (Bill Putnam's old studio).  Anyway, UAD has a plugin called OCEAN WAY STUDIOS which is a multi sampled IR effect done in different Ocean Way rooms with all types of vintage mics. Frank always sung with the band LIVE as you probably know so this approach has some logic to it.
 
If I was going for that Sinatra sounds for vocals, I would probably try this one first with a 47 or 67 mic (you get both the sound of the room with different mics).
 
Obviously, you can use UAD plugins in SONAR.
 
Jim

I found ReMatrix Solo and gave it a try and WOW, there is a pre-set called "Vintage" that is pretty close - lots of immediate mid-rangy reverb! I'll keep fooling around with it but it's pretty nice as-is. It took me a while to find ReMatrix as SONAR likes to give you more choices than you'll ever need.
 
Thanx to all for the help and especially to TheMaartian for the Solo tip.

Glad you found something that works!  I totally forgot about ReMatrix to be honest. I saw you used H-Reverb so thought you might have a larger Waves collection with IR-L/1.

Ah well many people don't realize IR-L comes with multiple GBs of impulses that you need to download separately. The default menu is like 6 general items. If you haven't installed them, definitely do, they're super useful. 

I lust for Altiverb, I've worked with it in a few studios. But yeah, it's expensive. And I honestly haven't had a situation that I really really couldn't do with something else.