Personal opinion but the only thing that makes any difference in sustain is the join between the end of the neck and the corresponding part of the body. Body mass really doesn't make that much difference but a poor join will and there is a simple way to make sure the join is at its best by something that I have done for years but most people here do not. People worry about the position of the neck screw holes that come from the factory and plug and redrill and again in my opinion a waste of time and believe me the hole position on some I have got are really garbage but nothing that cant be fixed. All I ever do with the guitars that needed it was to take the plate and turn over so the upper face is now on the body. Measure the hole with a drill or rule or calipers, what ever you have, clamp the plate in the position you want and drill the hole and the plate will guide the drill. Don't make the hole in the plate any bigger, if necessary just use a drill 1/2mm or 1/64" smaller. The only time you may have to plug is if the original holes appear outside of the neck plate but otherwise forget it. The screws are only there to stop the neck from hitting you in the face when you tighten the strings they do not position the neck or keep it aligned, that is what the pocket does. I also put about a 1/2mm chamfer on the neck base where it makes contact with the body so as it will fully seat on the bottom of the pocket so as to maximise the contact. I then clamp the neck to the body and use the same drill to make a small dimple by hand by twisting the drill in my fingers in the neck so I can use the right sized drill for the neck screws. The screws will have plenty of room for the next process. Assemble the neck and tighten the screws and bring the strings to tune. Loosen the 2 screws that are closest to the bridge, you can loosen them quite a bit as they are basically doing nothing as all the weight is on the other 2. Now just crack the other 2 screws maybe 1/4 of a turn and the end of the neck will now be hard up against the body as it will be draw backwards. Now just tighten the screws and that's it. I have store bought guitars that were in my opinion OK but once I did this the sustain basically almost doubled. Again just opinion but the most important faces to make sure that are in good contact is the end of the neck and its corresponding body part as it maximises contact but is a solid contact that basically makes the 2 pieces of wood as good as a single piece.