Spongy pedal after big bolt pattern conversion

Combination of air and drum adjustment.

To adjust rear brakes, tighten them up good until the wheels won't turn. Normally you would back them back off until they turn with "just a scrape."

But in this case, I'd tighten them up and LEAVE them until you get them bled, and get a good pedal.

I don't know anything about Viper calipers and cannot see where the bleeders are. The "simple theory" is that the bleeders MUST be in the top of the caliper, with no "air traps" in the way that they are mounted.

What procedure are you using to bleed? If you are using a manual pedal method, it is IMPORTANT not to release the pedal before you get the bleeder closed. It can suck air back in on the return.