The neck...

The frets seem to be the same position as compaired the the banjos i have here..

Keith Richards Rolling Stones guitarist plays his guitar with 5 strings tuned GDGBD

I would think fitting it to the round rim of the banjo would be most of the difficulty. The rectangular part where the four bolts go to hold to the guitar body is going to need to be removed, or something, and then finding way to attach the neck to a banjo's rim.

Bobby

You would have to cut off the part of the fingerboard that extends over the guitar body, and then build up a neck heel (like the modern Asian banjos do) that could be bolted horizontally to the banjo rim. I guess it could be done, but why?

Most people who do this seem to use them for banjo-caster electric instruments

Glue on a heel and then cut it to fit a rim.
If you get it wrong cut off the heel piece and try again.

I used to restring a guitar with light strings and use a fine gauge string (.008) on the sixth. I tuned it like Sonny Osborne tuned his six-string banjo. gGGDGbd

I’ve been wanting to do this forever… but haven’t found a round-tuit yet. If I was a good woodworker, I would make a heel block that mounts to the banjo, and then screw the unmodified neck to the heel block.

Also check out my unmodified telecaster, strung with 5 strings, a method I’ve been using for over 20 years, for hundreds of hours of silent banjo practice. I just eliminate the 6th string, and then use a .010 string for the 5th string. I run it through the 6th string nut slot, to get it out of the way, and make the open position finger more banjo like…. But at the bridge, the strings are normally spaced for proper picking.

If I were ever to put an electric guitar neck permanently on a banjo, I would not modify it, but would make a new nut for some nice wide 5 string spacing, I would run the 5th string full length, and then just spike it at the 5th.

If you want to mount that neck on a standard banjo rim, I reckon it could be done fairly easily-- you'd have to make a big old block of a "heel", fitted to the pot with co-rods or a dowel stick. It would have to be big enough to have a standard neck mortise routed into the top of it (in the right place to put the bridge where you want it on the head!). What wouldn't be easy is making it something other than real clunky looking and strangely balanced.

They could be turned into a lefty neck. Lefty entry level banjos are as scarce as hens teeth.

All my guitars have a high D string, rather than a high E string.

This used to be a six string. It plays quite well.