I have a 1989 G-30 with the 350 in it which has extremely low miles (38,000) but I am getting a huge puff of smoke on start up and only aftere the vehicle has been sitting for a period of 10 hours or longer.

I pulled the valve covers and took off the rockers and compressed the cylinders to hold the valves in place and the O-rings are pretty hard and brittle.

My question for today is would it be possible to not only install the new o-rings but also put in the umbrella style valve seals for adequate sealing of the valves without any issues?

 

IMO, you've found the culprit.

Instead of borrowing trouble by putting two parts where one should be, why not find out what style is used on your specific engine, then get the best replacement parts you can find?

I'm willing to bet one top notch set of seals will work better than two so-so sets.