When I was racing, I often used Copper Gaskets Unlimited in Phoenix, for custom gaskets, https://coppergaskets.us/
or you can google them. They do a nice job, and if you want to retain your thin .008 copper base gasket, the deck height equivalent would be to use their .032 head gasket instead of your .040. They have a ton of patterns in their computer and can make custom gaskets quickly. Their website says,
"Copper thicknesses we offer are:
.016" .021" .026" .032" .042" .050" .063" .080" .093" .125"
We also cut aluminum."
On the other hand, using no base gasket, and instead using Yamabond 4, is common on Norton
engines--I did that on my street Commando
, which seems very happy with the result -- but I also did experiment with no base gasket/Yamabond 4 instead on my old T140 race bike, and so long as you watch all the clearances, and keep at least .032 squish (mine touched using stock aluminum rods if you went any tighter), and adequate valve-to-piston and valve to valve clearances, and adequate crush (circa .030) on the pushrod tubes, it worked okay.
But, if you are taking the engine apart often, Yamabond is sort of a pain, since it requires some effort to get off the gasket surfaces with a scraper and solvent, but it will most probably not leak --I did have a few minor leaks doing this. On the other hand, a very thin coat of red silicone on both sides of copper gaskets (squeezed between your fingers until it is slightly transparent, like .005 or so, wiping of the excess) works fine--especially for race engines that come apart often because it is quick and easy to clean and you can do this at the track -- and seals very well. However, alone, with no gasket, red silicone can sort of walk out of high pressure joints like the base gasket and case seam after a while, so I wouldn't use it without a gasket.
Personally I had the best luck with, and I prefer to use, a base gasket with a thin coat of red silicone, instead of only gasket sealer, because you can be sure that with the gasket, it is more likely to seal any irregularities. And I had very good luck over many years using copper gaskets on the Triumph cylinder base surface, with a thin coat of red silicone. I've never used 518 to replace a base gasket, but the company literature says it is "medium strength", FWIW.
To the other warnings above about careful measurement of all relevant clearances, such as adequate squish clearance ( at least .032) and valve to piston and valve to valve clearances, and any possible ridge at the top of the cylinder, I can only add, when you are modifying deck heights, pay attention to the crush on the pushrod tubes when you dummy it up to measure.