273, or 318?

Dan the man said
What are the size of the valve's in inches? Is the length the same?

The 5.3 LS intake valves are either 1.9" or 2.0" head diameter. The Mopar and Ford valves are a slight bit longer. Ford guys have been installing the slightly shorter 327 Chev valves for years. Those are very close to the same length as the LS valves. Now I understand the 5.9 Magnum uses 8mm valve stems that are the same length as the LA valves. Swapping comes down to the cost of all the parts, valves just a portion. There is springs, retainers and locks. Also you may need screw in rocker studs and guide plates. If you swap out the 3/8" valves for the Magnum valves you will save weight. The Magnum valves have 2 keeper grooves like the LA but the smaller diameter requires keepers and possibly retainers. Depending on what springs you have and the cam, you MAY get away with reusing those. Compatability with the keepers has to be verified before trying that.
Because the LS are still in production by the millions, parts are inexpensive and readily available. Parts through the dealers are normally pricey as compared to as good or better quality parts from say Summit or Jegs. That depends on manufacture. Most stuff is cast or forged in China these days. Some rebox into their boxes and ship. Others perform QC checks and finish machining in the US. Probably a better quality product. The Japanese products we got in the late 50's and 60's was of poor quality. But the Japanese did customer satisfaction surveys and took what was stated to heart. The Chinese are where Japan was 50 years ago, but getting better. I do have a concern on Chinese stuff. First is that purchasing Chinese manufactured supports the CCP and PLA as any major business is tied to the communist party. Second is they come here, western countries, and buy up resource companies including farm land. Generally they employ Chinese citizens that supposedly are immigrants. Now if you want to set up a company manufacturing a product, you must first form a partnership with a Chinese entity who holds 51% ownership in the company, while you pay most or all setup costs. This entity whether a corporation or individual will be closely affiliated to the communist party. Once they see the product and processes involved, they will set up another company and produce tbe same product or very close copy for less money. Intlectual property theft, and you have no leg to stand on. Patent law means nothing to them.