In theory, images built for the 3B should work fine on the 3B+ - the SoC is the same (but with a clock speed boost). You have a choice between building aarch32 and aarch64 images - make sure your config.txt is configured such that it will boot the kernel in aarch64 mode if you decide to boot an aarch64 image ('arm_control=0x200' etc.)

There are some differences between the 3B and 3B+ as far as other hardware on the board goes (i.e wi-fi & usb/eth chip), but we don't provide support for that.

Regards, - Seb