11/11/2021 update: I incorrectly drew the volume pot's J113 pinout, should have been DSG, not DGS. I was able to use a 2SK246 instead of the J113 and flipped it 180 degrees. This jfet is wired up as a buffer so the 2sk246 should be fine here. I'll redraw the layout with the proper J113 pinout. Sorry for any inconvenience.
With the main board turned counterclockwise, should fit in a 1590BB.
The boost board should fit between the two stomp switches. Tap the power and ground from the main board.
11/11/2021 Corrected 2SK246 label/orientation (original jfet is J113 DSG, new layout down below)
And to use the boost independently like the original, wire it up something like this: