Bass guitar neck alignment

Hi all, got my new kit, the Flying V bass, and started putting together my first guitar. Measured and placed the bridge, put the neck into the body and drilled/screwed it in. It was a tight fit, and have checked since. It’s in there well. Then I placed in the tuning pegs and strung the bass. The action is really high. I dropped the bridge saddles. As Far As They Go, but still it is real off. This is when I took the neck off and inspected. Put everything back on, and checked the curve of neck. It seems straight/ meaning no concave or convex. I let sit and strung for 24 hours, but no noticeable difference. The nut seems to be fine- action is good at top/head of the neck, yet high at the bottom by the body.
I’m not sure what do do here. Did I get a bad neck or body? Should I start sanding the body neck slot to try and modify the angle of the neck and body placement?

FYI- although this is my first “build”, I have been playing for 30+ years and have plenty of neck adjustments, replacements, body/pickup/headstock, FrankenGuitar modifications.

It does sound like a neck angle problem, but some pics would help, if you can.

Seems like a classic shim job is needed....

I'd be cutting a 1cm wide strip of plastic from a 2L icecream container lid as long as the neck is wide, and insert that under the very end of the neck between the neck and the body... reassemble and check your string heights again and see where you are at. It is as close as a home builder can get to what many manufacturers and luthiers do to correct similar issues.... You may even need two such shim strips.

^^
Apart from that you'd need to insert the shim at the heel end (not the body end) of the neck pocket in order to get the neck pointing down towards the bridge more.

A shim at this end of the pocket isn't desirable IMO, but it will give you an idea of how much the neck angle needs to change by. I'd then re-angle the bottom of the neck, but it's not an easy job to do well and keep the bottom flat without curving the edges.

So you may want to stick with a shim, but it's going to be visible at the heel end, whereas one at the body end won't be visible.