Ice Freezer Conversion

I am working to replace an R-12 condensing unit on an old (30+ years) chest ice cream chest freezer, size 8ft L X 2ft D X 2ft W.
My dilema is what refrigerant to use for the new condensing unit. R134a is not very efficient at the minus 10degree F that I want to maintain, I believe R404a is too high pressure for the existing evaporator and tubing. I am thinking about going with 401a unless I can get some other suggestions.

Any ideas

By the sound of it, I would be going with R409 because it is a direct drop-in replacement for R12 with similar pressure/temp relationship without the need to flush systems or change oils. Having said that, I have been out of the commercial refrigeration game for a few years and might be out of the loop a bit now.

Good Luck

Dan