I know front 914 calipers are 3" bolt pattern as are 911 M calipers. Can you mount the 914 caliper on a 911 strut using the early solid rotor? Or does anyone have spare solid rotor I can try this on. I have a lots of late calipers but no early 914(vw type 3). Just trying to keep stock brakes with the 5 bolt pattern.

914 - 42mm piston
911 - 48mm piston

You can. It would have to be an early 914 caliper. The reverse is true as well... a 911 M-Caliper w/o spacers will bolt to an early 914 strut.

I would imagine that one going 5-lug would be able to keep his/her early 914 calipers if they ran a solid rotor. This would retain the stock brake bias as well (about the only upside I see).

There's just not much of a reason to do either configuration. Both use the#31 pad size, just different pistons. I think DaveP ran into a guy in Toronto (toronto901) that had 914 calipers on his 66/67 911 and didn't realize it.

Yeah, I took a pair of early 914/4 calipers off of a 1966 911 because a caliper broke at the bore so the O-ring was visible. I had a proper set to put back on.