Hamish, here is my 2 cents worth based on what I've learned over a very long time:
Rivet spacing will depend on the application as well as the "rules". If you are using 1/8" blind rivets (assumed since you have black Cleco fasteners), the spacing could be as close as 1/2" in high stress areas while a relatively low stress panel could be 1" to 1.5". That is a lot of rivets but distributing the stress over a wide area is what makes the joint strong. Even low stress panels need enough rivets to keep it lying flat against its rear tubing support. Look at how most airplanes are built- lightweight but strong and fatigue resistant. Lots & lots of rivets.
The prevailing rule of thumb says that a 1/8" rivet is OK for panels up to 1/8" thick. Most automotive applications are in this range.
Regarding types of rivets, there are two main types, solid rivets and blind rivets. Solid rivets are cheap but require access to the backside of a joint so that a bucking bar can be used to bulge the rivet into place. Obviously a solid rivet can't be used to fasten a sheet metal panel to a tube so we use the second type of rivet- the blind rivet. This requires access only to the front. There are many factors to consider when selecting blind rivets. Assuming you've already decided on the rivet diameter, the next consideration is the grip length. This is the total thickness of the material that the rivet will pull together: the sheet metal thickness + the tube wall thickness. Countersunk (flush head) types are used to achieve low aerodynamic drag or to present a smooth surface but the "universal head" type is by far the most common type.
Rivet material will determine its shear strength (usually more important than tensile strength) and corrosion properties. Aluminum (5056) body blind rivets are the most common types encountered. The stem material varies. Other rivet body materials are available- Inconel, Monel & A256, are higher strength alternatives. The type of construction will pretty much determine its cost and its fatigue properties. Structural rivets are usually designed to retain the pin stem within the rivet body after it breaks off. The retained stem greatly increases its shear strength and is resistant to vibration. "Hardware store" (Pop) rivets do not positively retain the stem so they eventually wind up being only a hollow tube, offering only low strength and a path for moisture to enter the structure. Not good.
OK, Pop rivets aren't good but aircraft structural blind rivets are hideously expensive (some even require special installation tools), so what do we do? Well, there are a few choices. Easiest is to use AVEX blind rivets. These are "semi-structural" aluminum body rivets with a steel stem, having a very wide grip range, and are installed with ordinary tools. Fortunately, they are inexpensive in decent quantities. Far more expensive (around $1 each) are rivets such as CherryLOCK and CherryMAX as well as equivalent ones from Huck, AllFast, etc. Buying these new are out of the question for most of us but there are sources (eBay, etc) for surplus stock that can be very reasonable if one shops carefully. It helps to become familiar with various part numbers so that you can weed out the stuff you can't use. Avoid rivets that require expensive installation tools.
Also consider the type of sheet metal you use for your panels. Aluminum is the most common, of course but there are a variety of alloys to choose from. All are almost identical in weight so the choice devolves mainly to shear strength, workability, and corrosion resistance. Weldability is usually not a factor but it could be. The highest strength aluminum alloys are 7075-T6, 2024-T3, & 6061-T6. Corrosion resistance and workability is usually in reverse order- 7075-T6 is really strong but it corrodes more quickly and it is extremely difficult to bend. Two alloys offer all-around good properties: 6061-T6 and 5052-H32. I chose 7075-T6 for the stressed bottom panel on my chassis (not a GT40) but I live in southern Arizona in the desert where the humidity is very low (it was 8% yesterday) and I protected the sheet and tube with a 2-part epoxy zinc chromate primer with a silicone sealant. If you live in a high humidity climate something like a 5052 alloy may be a better choice. Really soft alloys can be used but you will get no or little strength benefit from the panel- it will be strictly cosmetic or to keep the wind and dirt out. Why not take advantage of a stronger alloy and good rivets?
Rant over.
