How can I clean up a used set of roller lifters and pushrods? Can I soak them in gas or is that a no-no? Should I soak them in oil before reinstalling?

Using gas is a no-no (flammable) and it doesn't work well. If you're going to use a no-no, use mineral spirits or laquer thinner (equally dangerous).
Or you can use what I use: a spray can of "brake parts cleaner" (non-flammable). The parts will have to be quickly put in oil to prevent rust as the solvent will remove every trace of oil from them as well as any dirt, grime, sludge and varnish.

Since when is brake kleen non-flamable? We used to make flamethrowers out of them at work!

And I wrote my name on the garage floor once with brakleen and then lit it with my cigarette, and that **** all went right up.

Ever since they figured out that burning the crap out of mechanics was a bad thing.
See the attached picture enlarged and notice the can on the right is one I keep just to remember the "good ol' days".
Then notice the ones on the left...

yeah every shop I've ever worked out the brake clean is plenty flamable.. can't say I've ever heard of the other stuff.. good suggestion though.. I think I'll do just that. FWIW the brake clean we use at the shop I currently work at is still the flamable stuff..
Mineral spirits don't really clean all that well.. I already washed them in the parts washer and thats what's in it.. spit works better half the time.. lol

Rest assured, mineral spirits (AKA-"paint thinner") is not in your parts washer. It evaporates so fast that in a week all you would have is some sludge left in the bottom.
The parts washer (if serviced by a company) contains a super safe, lame-*** solvent that doesn't evaporate, it's good for getting grease out of bearings and that's about it.
A quick history on Brake Clean, Brake Kleen, Brake Wash, and Brake Parts Cleaner.
In the beginning, (25 years ago) there was gear oil on brake shoe linings.
People figured if you could get the oil out of the linings it would save money.
Someone found that "dry cleaning fluid" (trichloroethelyne) worked like a charm. They put it in a spray can and Brake Clean was born. It was non-flammable and would render oil soaked shoe linings looking like they just came out of a box new. It was a miracle. Unfortunately it was a "chloronated solvent" and did not break down. It started to show up in drinking water supplies and is damn-near impossible to remove. So it got by the government.
(notice the can on the right in my picture says "non-chlorinated solvent")
The mechanics were sad.
They came out with a product that worked almost as good and called it Brake Kleen (Brake Wash). It has the minor drawback of being extremely flammable and does indeed make a dandy blow-torch, but does not make explosive vapors like gasoline.
The mechanics were happy again.
...except for the ones that caught themselves on fire. They were sad.
The engineers went back to the drawing board and Brake Parts Cleaner was born. A non-flammable, non-chlorinated cleaning solvent that works like "steam-cleaner-in-a-can".
The mechanics were really happy now, especially the ones that didn't burn themselves up.
Nothing will ever be as good as TCE (trichloroethylene). And yes, I got to use it before it was . I miss it, but really like having non-toxic water to drink.

where do they sell the non-flammable stuff, maybe its a certian brand? Just looked on a can I had lying around, still says warning flamable.. also says that it cant be sold in california because something in it causes cancer in california.. sure glad I don't live there.. I've been using the stuff for years..
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Just cleaned the lifters with it, worked ok.. not so well on the pushrods.. kinda gummed up inside.. maybe a pipe cleaner will do the trick..

As near as I can tell, the people in California must eat everything they can get their hands on. It's pretty bad when you buy a DVD and it has a California warning label. I think their state government just wants to make sure everyone in the state is neurotic.
As far as the brake parts cleaner goes, the non-flammable stuff came out in the last 5 years. I get mine from work but I'm sure the auto parts stores carry it in a different brand. Just checked. Advance Auto has "Advance Auto Brake Parts cleaner", Valvoline Brake Parts Cleaner(after 2004) also canned as "Pyroil" is non-flammable. Napa has the Valvoline stuff in their NAPA can. Get this: The nuts in California are stuck with nothing but the flammable-as-hell CRC stuff!. The dumb masses are fortunately located a safe distance from us sane people...