So far, typical BBC stuff... except the head bolt, that has me a bit concerned. I might consider cutting the head of the bolt instead of welding on it. Pull the head and worry about the stud after you get the tension off of it.
Surface rust on the pushrods is not an issue. If you're worried about it, a scotchbrite pad will take care of it. Let us know if you find any other rust, especially in the cylinder bores or lifter bores. Very unlikely to find rust in the lifter bores, but possible, and that bears further digging. The only time I see rust in lifter bores is when coolant has mixed with oil and the water makes rust happen where air can't typically get to.
Inside the rocker arms where the rocker ball lives should be nicely polished and not galled. Both ends of the pushrod should also be shiny polished and have no gouges or scratches. Lay all 16 pushrods on a flat surface and roll them to make sure they aren't bent in any way.
Valvespring compressors are cheap like this one. They are just a simple prybar lever that hooks under the rocker nut and you push down to compress the spring. Push it down and gently tap up on the valve and remove the locks and retainers.
You likely already know this, but it is wise to keep all valvetrain components in order for re-use, especially lifters if it is a flat tappet cam. I like to take a piece of cardboard and stab the pushrods through in the same order as they came out. Valve springs can go in a labeled egg carton. Given the super cheap price for replacement parts, it might also be wise to just get new pieces. If you're still planning on a cam swap, never re-use a flat-tappet lifter on a new cam or vice versa. Since you have a pretty wildly mismatched cam/compression thing going on, I don't think this is an issue. I would take the cam and lifters and throw them in the trash if the cam actually has the specs we discussed before. Or make them into a neat piece of art. Or keep it under the seat as a way to protect yourself from attackers.
This is premature, but I'll post it while it's on my mind. When you get to re-assembly, talk to me. There are some things I've learned over the years and (like I said, I'm a bit rusty) but this could be important: Assembly lube. Use it where it is needed, but don't use it where it isn't needed. Assembly lube has some bigger particles in it which get caught in the filter. It isn't common, but I have seen new assemblies go belly up as the assembly lube gets washed into the filter and the bypass valve lets unfiltered oil with lovely machining bits get pushed through the engine.
There are two types of assembly lube which I lovingly call A) death paste and B) strawberry syrup. Death paste does not really mean "death." That is a hyperbolic description I use from a story of an engine I bought once that was slathered all over inside everywhere with the grey assembly "grease".... the cylinders, lifter valley, the entire crank... it was like two full bottles of the stuff was all over the place. If I had started it, it likely would have hydrolocked. If it even fired, oil pressure would have likely never developed, and if it did, the filter would have clogged right away and bypassed allowing all the fresh machining leftovers to flow to the rest of the engine.
Strawberry syrup has far few solids and a little goes a long way. It is a slimy liquid as opposed to a thick paste. My general rule goes like this: Any journals, bearings, and bores get fresh motor oil. Period. Dip pistons in oil before installing. Soak lifters in oil overnight before installing. Rod and main journals and bearings get oil. Any parts that are a direct, high-pressure metal-to-metal get strawberry syrup. That means the cam journals get oil, but the lobes get assembly lube. The base of the lifters, tips of the pushrods, rocker balls, and valve tips get assembly lube.
The only time I would use the paste instead of the syrup is if you know the finished assembly might sit for a year or more before firing.
Get yourself a cheap oil priming tool. Run it for about 5 minutes before startup. Once it starts, you can stop it for a change; carb adjustment, timing adjustment, but the most important thing... can't stress this enough... RPMs must stay at 1500 rpms or more for 20 minutes. The cam lobes are entirely lubed by oil that gets thrown on it from crank counterweights. If it idles during cam break-in, it won't get enough lube during that critical time. Flat-tappet cam break in is misunderstood by many, but I'll explain.
Cams are made from cast iron which is relatively soft, then the outside .005" or so is induction hardened. Same for the base of the lifters. So here's the metaphor I use for cam break in. Imagine you have two sheets of 400 grit sandpaper on two sanding blocks. Rub them together for a long time. Both pieces of paper will give up their abrasive material at similar rates until they work themselves smooth. In the case of cam lobes and lifters, you can imagine the need for higher rates of lubrication from higher RPMs as this happens between two pieces of iron.
This is also why you can't put new lifters on an old cam or vice versa. In that metaphor above, imagine your two smooth pieces of paper after break-in. Now imagine replacing one of the pieces of sandpaper with a fresh sheet of 400 grit. It would tear through the old piece of paper in a heartbeat.
I wish I were a little closer and had more time. I would love to build an engine with you.