The original "slapback" tone was often achieved with analog tape delay. Where the machine was recording the signal and playing it back at a set delay time. So, you would get a "slapback" of the notes you played but they wouldn't continue to trail off. If you don't get a decent slapback tone from the Slap mode... try the "Tape" mode.
The "feedback" setting on your pedal will control your trail off time, you'll want that set fairly low. You'll want your delay time set fairly high. You'll want to use tap tempo to set your slapback tone otherwise the delay rhythm will end up being out of time...
As for the reverb - If you have a spring reverb, throw a little of that on it and you should be pretty dang close to where you want to be. I have a Vox AC-15 and I add a little bit of tremolo and it sounds pretty good.
Hope that helps.
