The issue is the neck is 25.5 inch scale. The plans are 25 inch scale.
Umm, isn't this dead backwards? The Carvin 6-string neck is a 25" scale, and the PLANS are for a 25.5" scale guitar, right? :icon_scratch: ???

I'd get that figured out...
http://www.carvinguitars.com/necks/guitarneckthrough.php
There are a few good ways to reduce the psychowillies, at least mine. You are looking for an accurate AND attractive bridge placement, pickup placement, body cutaways
vis-a-vis the fret locations. So you get your 36" long construction paper, or painter's big pad, or light cardboard from... a really long thing. This is the neck. Pop on a centerline. Draw a headstock, looks like for a Carvin is a tapered box, 6.5" - 7" or so - NOT critical. Then draw the nut to bridge, down the 25" down the centerline. CRITICAL... :laughing7: the nut's going to be 1.6875" wide centered on... the centerline! The bridge's STRING width is going 2.125 centered inches. Halfway down the neck is the 12th fret, at 12.5", and another half from that towards the bridge, is going to be the 24th fret, 6.25" PLUS 12.5" means the 24th fret will be 18.75" from the nut. OH LOOKIE!
http://www.stewmac.com/freeinfo/Reference/Calculators/i-fretcalc.html
Your fingerboard is 18.75" long! Rough in the sides of the neck, draw a little bridge picture, then cut it out like paperdolls. Barbie took the brown acid and turned into a... guitar neck?!? I personally would keep the paper behind the neck, Carvin says 13". 13+25+7=45, you could knock 7" or so off the bottom. Draw in some frets, it wouldn't hurt to try to be accurate, at least at the top 6 or 8 frets.
The rest is obvious, right? You take your other big piece of cardboard, I'd guess without reading you want 14" X 19" - centerline, draw and cut out the body. What you have now is the ability to try all different degrees of how deep the neck is set into the body. Can you only play the 24th fret with your little finger, how big are the cutouts on the treble (and bass?) side etc. Do the pickups look right... etc.
~ YOU DO NOT - EVER, EVER, EVER WANT:
"Hey man, let's just LET THE SAW DECIDE WHAT IT WANTS TO DO"... man. ~
If you're still early in the design process, it's a lot faster to work (AKA "doodle") with carefully sized-down drawings. Half size will get a whole body on an 8.5" X 11" piece of paper, there's all kinds of graph paper that lets you dump down to 1/4 size, even 1/8 or 1/10 depending on the paper. It wouldn't hurt to follow a source plan to excruciating detail either, it'll be good practice for your kid, who sooner or later be venturing out into the world where people will be ordering him to do excruciatingly dull stuff. Best to hurry up on that rock star plans, kid....
the excruciators are waiting for you.... :evil4:
And Point #2 in the PsychoWillies Prevention Plan (P.W.P.P., we take Paypal!) is to have ALL the parts in hand before you start turning money into sawdust (mebbe a guitar, too!). There's really no point in drilling for the bridge, say, until you have it, can check through everything, measure 47 times etc. Even Saint Leo Fender would be envious, because you don't have to make a bunch of parts that will all fit each other. Just one.