Do I need frame mounts and motor mounts?
I have not done this. I expect you need both the rubber cushions (bolted to the frame) and the steel brackets (bolted to the engine) from a 305 or 350--I think they're the same. GM used steel brackets mounted to the engine on many different body styles--passenger cars, trucks, vans, etc. Be sure to get the ones for your application, 'cause they're NOT all the same.
As I recall, the frame mounts bolt to different holes in the frame than the V6 mounts.
Can the frame mounts be purchased new?
Effectively, no. They're available in the aftermarket, but they're hateful cheap Chinese junk using thin steel shells and questionable rubber.
There are Polyurethane cushion inserts available from at least two suppliers; I couldn't make the Poly cushions fit into the original steel shells. The design is totally different from OEM, yet both suppliers product looks the same.
The OEM mounts were discontinued years ago. OEM mounts are Treasure-Yard Only at this point.
You must be registered for see images attach
Energy Suspension Poly mount cushion vs. OEM rubber. Poly is wider, and has steel reinforcements at the wrong angle.
You must be registered for see images attach
What is needed just in regards to the motor being mounted?
I would not expect the V-6 mounts to work on a Small-Block, even if you move them to the proper holes in the frame.
Good luck getting the rubber cushions. When it was me in 2018, I put my 300,000-mile worn-out rubber mounts back in, and returned the Chinese crap to the store, and the Poly inserts to Summit. I was not happy about that.