Have you used carbon paper to check the fit? I rebuilt a "50s Kay that had major problems with the neck joint and found that the dove tail on the neck wasn't contacting the slot in the body in more than a few places. It's possible that there are only a couple of spot holding you up. While you are at it, make sure that you clean off as much of the old glue as you can get.
Other than that, fitting the dove tail will require whatever it takes. If the neck seems to big, or the slot too small, I think it's a lot easier to change the dove tail on the neck than the slot. I will warn you that I found that the narrow end of the slot on my Kay was "wallowed out so that the end of the neck taper got loose when I finally got my neck height correct. ( I've seen this on my Kay Kraft Mandolin too. ) I had a LOT of trouble getting the heel tight at the tip. In the end I still ended up with a slight gap there even though the neck is tight otherwise and the angle is fine now.
So, I'm not sure about this, but I think that when you correct the angle of the neck, you will have a better fit, since the angle fix will reduce the dovetail a bit. Of course, you can't get the thing together to determine the angle at this point, but I would assume that resetting the angle will make the fit easier, and then you can go from there. My 2 cents, smarter and more practiced minds will be better at this.
... or if I used the [apparently] last two brain cells I have left and rubbed them together, I would have remembered I have a block under the fretboard extension! That's why [Wes] you have that excess neck.
Sorry for the useless post here. Carry on.
@Ned: Good suggestion on the carbon paper. No that hadn't occurred to me as I will still need to do that. I have had to fix some chips in the dovetail and naturally the fit isn't 100% and have had to sand/carve [the repair] flush.
@Mark: Sorta. Somehow the neck angle had been reduced -- either through age/wood shrink/loose glue or perhaps the top had bowed inwards due to constant string tension. I dunno, but the bridge was adjusted all the way down (two-piece wood bridge with thumbscrews to adjust height). Even then, the string action at the 19th was WAY high. About 6-8mm high. The neck from the bottom of the heel was separating about 1mm or so. I knew I could take the neck off and either shim the neck tighter and reglue, or worse, lower the neck angle ~ .5-1 deg to compensate. I still think there might be some issue under the top but I can't get in to do any work as the F-holes are small and too far removed from the neck joint.
Never a wasted thread - it is good to think out loud, and I always learn something (thanks Ned for the reminder about the carbon paper trick).
That is a cool looking guitar. Why the odd angle of the bridge? Is it movable and just a bit out of place, or was this guitar originally a lefty? Looks like the intonation is going to be very strange.
While good neck setters are around, a bit of a different story.
For a custom built dreadnaught with bolt-on mortis and tenon, the neck hand shaped from a blank -- can a stock replacement neck be adapted to match the existing neck block? The neck is off the body at this time ready for a new set, and ready for a new fingerboard. A wider neck with lower profile is wanted.
There are not a lot of "stock necks" available and fewer of the bolt-on type, and most of the time are they made to Martin's specs. So the good question is : what's the size and geometry of the neck block pocket? Then get the specs of stock necks you can eventually find and see if they can match.
Needless to say I'm quite pessimistic : if you build the body and neck block to your own specs, there's not much chance you can find a industry-standard replacement neck that would fit. I think I would build another neck.
The neck is basswood (very similar to poplar, and often interchangeable in the lumber markets) and is a fairly lousy neck material, because it's so soft. You can do a great job of precision fitting the neck, only to find it pull up again because the basswood compresses so much. Is the neckblock also basswood? I reckon it is. This is and always was a neck joint born to fail. You might want to consider a bolt-on alternative.
What you have said here makes a lot of sense of the trouble I had setting mine correctly. I did it over and over trying to keep the joint tight. It plays very nicely now but it still pulled up just enough to leave a small gap at the base of the heel. If I ever pull it again, I think I will make a tool to bolt-on through the end pin hole so it will stay down next time.