There's no angle involved, per se. Whether you use wood screws as per standard or machine screws with threaded inserts, they both have to go in at 90° to the neck. It also doesn't matter if the body top is carved. But, in the case of a contoured heel, two of the screws are 1 1/2" long, and two are 1 3/4". Otherwise, all 4 are 1 3/4" long.
If you use threaded inserts and machine screws, the 90° angle is critical. If it's not perfect, the inserts will cross-thread and start a waking nightmare. This is why it's important to use a drill press and great care installing them. It's not hard to do; I've done it hundreds of times. You just have to be careful.
If you've got the inserts installed already, try threading in a screw to the neck heel before it's attached, and check it with something square to make sure they went in right...
If the screw doesn't want to go into the insert properly even with the neck off the body, then you've got other problems with the inserts. For instance, it's possible the hole wasn't drilled deep enough, or the worst case - the threads in the insert or bad for some reason. If that's the case, the insert will have to be replaced.
It's also possible the spacing of the inserts is slightly out of dimension with the spacing of the holes in the body. If that's the case, you may be able to just open up the thru-holes in the body a little bit. The screw shouldn't thread in the body at all - that's supposed to be a clearance hole. It'll be slightly larger than 1/8" (about 5/32" or 11/64"), so the screw will basically just about drop through were it not for the head.
If you need to have it done, PM me. I know a guy who knows a guy who does that sort of thing here all the time :laughing7: