Am enjoying this thread on the A/H 3000 build.
I am not sure I would trust a blob of low temp solder to plug a hole on a rocker arm.
There is high vibration and some residual
oil pressure conspiring to pop the solder plug out. Although solder is soft, I don't think I would want it floating around in my expensive engine build up.
At the least, can you get them BRAZED?...I was gonna say silver solder but brazing I believe is a lot stronger.
Best would be remove them, tap them, and put in a threaded plug. You are in pretty deep $$$, I would hate to see your effort compromised.
Maybe some of the machinists/engine builders reading have some ideas on the topic.
Have fun with the 3000, cool car...!
Pete