If the neck you get is compatible with it (i.e. the truss rod doesn't clash), I highly recommend using the Fender method of rounding the heel instead, or, better yet, the ESP method.
This is the Fender method. It is flat like a normal heel, and simply moves one bolt so the corner can be rounded off. You don't need shorter bolts, and everything sits flush.
This is the ESP method. It's the Warmoth and Fender designs combined, so one screw is moved backwards to cut a corner off
and the whole heel slopes toward the neck. This provides the very best access, but it's a harder join to make, you do need bolts of different lengths, and it's hard to get the bolts to sit flush with the neck plate.
The Fender one is done by simply filling in the existing holes in the neck heel and neck pocket, cutting one corner down, and drilling a new hole. There's no funny angles to mess with. You just need to be sure you won't hit the truss rod, which can be an issue with certain Warmoth necks. (And is one of many reasons why Warmoth need to stop using the damn silly side-adjustment.)
The ESP is, as you can imagine, a case of sloping the heel first, and then repeating the Fender method to cut off one corner and move a bolt backward. Again, your neck needs to be compatible, i.e. not using a truss rod side-adjust system.
If your new neck is compatible with either of those methods, go for those. Cutting off the lower corner improves fret access far, far better than simply sloping the heel, Warmoth-style. Making a Fender-style contour is also much easier and quicker than making a Warmoth-style slope.
And, though I wouldn't really recommend it, three bolts is usually enough. Fender use a 3-bolt join on many guitars. So you could easily simply ignore the bolt on the lowest corner, and round the heel any way you like, as long as the other three bolts are left more or less where they are; reducing to three bolts
and moving them closer together wouldn't be such a great idea. Use bushings instead of a plate and you should be good to go.
