Technical Flathead valve procedure

Thought I was being smart, rebuilt my 48 V-12 with 8BA valves, guides, springs, etc. By some miracle, the engine started, but was stopped right away due to excessive mechanical noise. Pulled the intake to adjust the lifters. Really annoying as the cam was reground for solid adjustable lifters, and the adjustment is way down in the lifter boss in the block. So I use a Speedway pry bar to hold spring up, put a 3/16 bit in the hole we drilled in the lifter boss for the purpose to hold the lifter, and can just barely reach the adjustment screw head with a box wrench that has a slight tilt.
Was doing OK until I got to no. 5 intake valve when upon prying, the valve locks that hold the valve in the retainer came out, and disappeared into the engine. Got some more locks, but can't figure how to install with the valve assembly in the motor. Anybody have a technique, or do we need to pull the head and remove the valve assembly?
Thanks
Abe

 

Don't know if you can apply air pressure via adapter to spark plug hole or stuff rope in there(leave an end hanging out to remove) to hold the valve closed, but normally a headless block has the assembled valve, spring, keeper, etc pulled down through and the lock ring put on below.
Good luck/

 

Can you press on the top of the valve through the spark plug hole? I know on V8's the plugs' location varies and sometimes you can see a valve and some not.

If you can hold the valve down, you should be able to assemble the rest of the spring and guide and keeper stuff from below with a really agile and small set of hands. Do you have agile and small hands?