What should my final whole size be for the cones and end cap on a .308 suppressor build? I've read where people ar using .340 to .375".

My other question is, can I change my form 1 to indicate .223/5.56? I originally designed this with the idea of using it for .308 down to 5.56. I could still. But I want to get the best performance I can out of it and if reducing bore size to accomodate a .223 round helps, I'd rather do that. I can always get a .308 suppressor later.

What measurements and advice can you offer?

If you haven't built the suppressor you can notify the ATF of changes. Keep a copy of your notification letter as their acknowledgement response may take a while.  If the Form 1 said .30 cal conceivably you could bore down to 0.300" and your Form would be technically correct.  But it's a bad idea to mark a pressure vessel inaccurately to its purpose.

Good clipped baffles in a .30 can still suppress 556 well and will gave lower back pressure than a 556 only bore.  Bores of 0.360 to 0.375 are in the tighter side of commercial suppressors. A front cap exit bore of 0.405- 0.410" is typical, particularly for  longer tubes.

How well you can drill or bore concentrically is another factor, as is the squareness of your mount. Opening each baffle up a little more as you move toward the front cap can prevent baffle strikes.

Got it. I haven't built it yet. I do wish I had done more home work though. I was just in such a hurry to get my first tube I didn't think about everything. I just watched a couple of Youtube videos and ran. In the future, I'll either find a set of reputable plans that are effective or buy a tube from a dealer.

Can you or anyone else tell me if I need a space at the end of the tube, or can I put a cone at the end of the stack? If it matters, it is a 60 degree skirtless cone. I can get a 6th cone in the tube, but it will be at the end of the stack.

If you don't have a lot of leeway on your blast chamber spacer to adjust stack length, then a spacer at the end is a good place for that adjustment. You always want the mount threaded tight and square to the tube and your front cap not seated by maybe a 1/16". That way if the stack compresses on firing  you can tighten it down with the front cap.  But, if you are using a tubing cutter, they need a minimum length to get purchase on, and trimming small amounts works better on a lathe, or careful sanding on a belt or disc sander. It's easier to do that on the bigger blast chamber spacer, just to get a hold of the work piece.

For short tubes the extra baffle tucked up to the front cap may be worth it.  Something like an Omega 9k is configured that way. On longer tubes a little extra expansion space before the front cap is typical. Keeps weight from the end and by then gas is mostly slower and expanding on its own without help from baffle features. Without a meter you may not a hear a difference.