Replacing

1- confirm that you have a mechanical cam, pull a valve cover and see what you've got.
2- do you want a mechanical flat tappet cam? because you can change it if you so choose, there's really not much to it considering you're already doing a cam. just a consideration.
3- heads/valve size are your limiting factor, basically.

a 273 isn't going be a torque monster, but the stock 2V bottom end should have enough compression to support a reasonable cam. because of the relatively small heads/valves/ports and factory configuration you're going to be somewhat limited on size. but it doesn't sound like you want to go full ham, so there should be several that fit the bill.

the isky E4 cam is a popular choice. it's basically the commando piece with a little more duration. a bump up from there would be something like a stock 340 cam. anything much more than that and you'll probably want to start thinking about converter stall and rear gear. probably about the most you could go without upping the compression and some better heads would be something like the comp 20-246-4

no matter what you go with, i'd recommend upgrading the springs.