Choice for EDM solid flat tappet lifter.

You can make your own EDM lifters with a Dremel & cut off wheel......on any lifter, sol or hyd.
Use a .020-0.025" wide cut off wheel. Grind a groove from the oil band to within about 1/8" of the lifter base, about 0.015" deep, not critical. Been doing it for 25+ yrs.
If you don't want lifter failure, try to find a set of 25+ yr old lifters, new or used. If used, have them re-faced. Not sure if slant 6 sol lifters can be used. I have also used hyd lifters by using spacer under the piston to make them solid. Grind a shallow groove or flat on the piston top prevent any chance of it pumping up.