Yeah so..
On the original rd400 rims, cast Alloy, can i change the valves to go tubeless or what do you recommend?
You should run tubes. Tubeless rims have a different profile that will hold the tire at the edge of the rim if it loses pressure. A tube type rim will allow the beads to drop into the well. That makes the bike unstable.
By 1980 Yamaha was fitting similar rims that were designed for tubeless tires. There is cast in lettering on one of the spokes saying something to the effect "OK for tubeless use". If you don't see that text it's risky to go tubeless.
Ride safe.