I never have problems with bubbles. I always pressure cast. I mostly do two part molds. Some cheaper resins have a tendency to naturally produce bubbles. Careful, Some resins will foam up when you vacuum. I use Smooth-On products for all my casting needs.
I use automotive quality greass and wax remover. 3M is a good brand. But Turtle Wax sells a bug and tar remover at the parts store. Same thing. Removes all wax and sillicon. Just wipe it downd with a well soaked rag. Do this before any sanding is done to you parts. Use it again before you paint. Removes oily finger prints.
I use Smooth On too. The way I make my molds is differt from what others use here. When I pour my molds. its a closed mold. Meanning I pour resin into the mold threw funnels. In the mold has runners and gates. Any area that I see that will trap air. Such as the rim of your wheel. I cut a vent into the mold to have somewere for the air to go out of the mold. I use a peice of .072 stainless steal tubing to punch a small hole into the top half of the mold. It has to bee tubbing. As it goes into the mold. It will cut a small core out of the rubber. I push it all the way threw to the top of the mold. This way the air and resin will flow freely into the mold. Eventhough I use Smooth On 310. It is a no air bubble resin. I still use a pressure pot for curing. no more than 35 psi is need to collaps and bubbles trapped in the mold. Useing this method. I never have any air bubbles.
Most early SBC are all the same in outward apperance. Only differences are valve covers and air cleaners. There are many resin castes that sell only the valve coers and air cleaners. The valve covers in the picture you posted are early vett covers. Air cleaner early BB maybe 66 - 68. Kit bash for the engin and find a caster that has the valve covers and air cleaner you need.
Ann here is a cheap way to make mold boxes. I use Glade brand food containers. They come in different sizes and usually cheep. I use them because on the bottom it is marked with the volume of the container. The small round one is about 3 oz and the other is 24 oz. This is helpful in calculating volume for rubber. Also you can just fill it with water and pour it into a cup that you will use to mix your RTV in. I use Smooth-on 1:1 ratio. They have mix cup that have measurement markings on the side! I cut the bottoms of then off. Then invert them and use a hot melt glue gun to glue them down to either a piece of styrene or I use a piece of melamine coated shelving. You can get that at the home centers. Both containers and board are reusable after you pour your rubber. Container will come off the rubber will little problem as it will not stick to the rubber! Just peal off the hot melt and your good to go again. Nice thing about food container is you can get them in different sizes.
This may sound crazy. But when I worked in the business. We used to put parts in the oven on a flat cookie sheet at 130 degrees and leave it for a few minuets. All plastic has a memory. So heating it relaxes the plastic and it will return to it’s shape. Pull sheet out and let cool! Try to support parts so they will not collapse. Some times putting them in the sun helps. Just keep and eye on the parts.I have protyps that I cast years ago. I keept then in a suitecase. I opend it up and some of the parts were deformed. I just let them sit in the sun and they went back to their original shape.
I have to agree with Art. I myself was into commercial casting. Doing prototype products for the design industry. I used Vaseline exclusively on all two part molds. Never ever had RTV stick together. And It didn’t matter how long I wanted to pour my RTV. The only time I would thin the Vaseline is when I used it close to the part. Otherwise I used it just as it came out of the container. Used a good brush to brush on. Don’t worry if it has brush marks. It will not matter to the mold. Plus like Art said. Its cheep and easy to get! If you get some on the master, don't worry either. You can take a Q-tip and whip it off. Then when you’re ready to pour your second half. Use spray mold release on the master. Then pour. And Richard, all is not lost on your first mold. You can salvage that mold and part. If you have cut it out and did a careful job. What you have created is a book mold. One shop I used to work at. Did these for all their molds. But we used a translucent RTV. We could see into the mold somewhat and cut the mold open. And as far as rubber and urethane. I use Smooth-on. I am just getting back into model making and I am fortunate to live in the Denver area. I can place my order and pick it up locally with out shipping charges. I have found them very helpful. I can call them anytime and get answers. You may have to call the closes dealer to you. Mine is Reynolds Advanced Materials in Denver Colorado. Smooth-on is much cheaper that Aluminite! I went to my local hobby shop and Aluminite kit cost $35. for the RTV. And its only a pound and half. Smooth-on is only $25. Since its is on my way home from work. I just pick it up. Look on Smooth-on web site for a dealer close to you. You can save shipping cost.