I think the two clamp method would probably work, but there is at least a slight chance that it could crack the body wood if you are not careful. No chance of that with veneer.
...but speaking of body wood. The pics make it clear that it's an open grain wood. So not basswood, alder, maple or poplar. It looks like ash to me because it's light and has pronounced grain. Good that you ordered some filler ;-) You wont' need it on the neck, but you will on the body.
The neck never sits straight with respect to the body. It always sits on a slight angle. There is a slight downward slope at the pocket So a single clamp at the cavity will definitely lift the neck up... I think the clamp has to be compressing the lower part of the neck with the back of the neck pocket like this
As for scale length... Inside of nut to pick up end of saddle with saddle at 75% in bridge in High E should be the measure of scale. Bass sides are always a bit longer to compensate for thickness of the strings. People do measure nut to 12th and double up. But that could be off on the bass side
A wise man once said measure thrice and fix once. Pls do not be in a hurry. Pls do a full mock build and check everything including alignment and angles and pickup pole positions and then take a call.
Hooray! Build... DONE. Well, almost. Finish completed yesterday and then polish, soldered the pickups, pots etc. and fitted the hardware today.
One problem that almost scuppered the whole thing though... the high E string bridge post is slightly askew which means I can't lower or raise the bridge much past the lowest point*. The guitar seems ok after a first play but I've not properly checked the intonation yet.
However, I'm really, REALLY pleased with the end result. Thanks so much everyone's invaluable help over the last week 🙌
Ash Explorer
ToneRider pickups (neck Rocksong, bridge Octane)
Grover tuners
Danish oil and beeswax finish (after natural grain fill)
* when the bridge posts are fully lowered and rigid, the floating bridge can't fit over both posts - raising the posts slightly gives some wiggle room but it's too tight a fit. This wasn't a problem in the mock build .
If you used the method that lots of us do...where you just wrap the screws masking tape rather than pushing the ferrules down into the wood, it could be that there was enough wiggle room that you didn't see the problem.
With a nice finish on it, you probably don't want to plug and re-drill...although it would not show much since it's under the bridge.
The only alternative I can think of is that you may be able to use a rat-tail file to widen the hole in the bridge where the post goes.