Is it safe to crack open the coolant drains on the engine block on a 23 yr old truck?

Ya know ... a long long time ago with older cars (when those cars were younger and so was I), I used to use a product that is still available today. I've never owned a 1st gen Tundra, but a '90 Jeep Comanche was the last vehicle I used this on. It worked pretty well at flushing everything out of the cooling system, but you end up with tap water in there. So if you go this route, then it's a good idea to run several gallons of distilled water through your system before topping with full strength antifreeze. The "T" fitting goes into a heater hose that you cut and leave it there permanently. Anyway, instructions are in the link. https://www.prestone.com/product/prestone-flush-n-fill-kit/

Edit ... after reading the part that i thought would explain the installation, I realize that is doesn't explain it. But the instructions are definitey with the product itself.
(Hint and disclaimer: you attach a garden hose to the "T" fitting installed in the heater hose, remove the radiator cap and turn on the water, there's a diverter that you insert in the radiator fill neck and you can attach a hose to that to direct the flow of the antifreeze that's leaving the radiator. It helps to completely flush the old out if you idle the engine during the process and turn on the heater to flush the heater core. When I was young, we didn't worry about environmental stuff. Today it might not be all that practical if you have such concerns. You pretty much have to run the water till it comes out clear.)