Most of the SE/30 cards are vertical due to the layout of the motherboard and case. Someone else, I'm sure, can give you details, but you won't be looking at LC PDS cards and thinking they could go in an SE/30.

Regarding m68k GNU/Linux, please do let us know how far you get with it. The last time I checked the state of Debian on m68k was pretty poor due to a lack of up to date toolchain parts. If they ever do get things worked out, I'd love to give it a go.