If that shows says it's for food. Just smaller than some like it for A/C.
What should vent temps be you ask? It can vary by all conditions at the exact time you check.
I like faster digital wired remotes you can put down inside a center vent. Almost anything should get you low to mid 40sF depending on fan speed requested from actual air temps outside of about 75 - 100F or so. Humidity, sun light and how long you've run it all count.
Some very large vehicles or some just lousy designs (not too many) you'll only see 50sF. If you can't get into the 40s with some regularity something isn't right. This is a 1991 which just age you can forgive some efficiency loss of plain dirty exterior of parts and some air leakage around items that should seal tighter.
Converting doesn't really change output temps but does change the BTU or volume it can continue to put out lower temps when stressed as it's only 80% the caloric value of R-12 from the beginning. Components are made to exchange heat that 20% more to compensate if all correct.
In a small cab of a truck the original was most likely overkill so shouldn't notice much difference except I notice converted vehicles are a bit slow to achieve max performance.
I'd get a thermos that's larger and meant more for A/C than the one shown just because. Touchless or infrared can be funky with checking non solid items probably best to really get used to each one and their quirks first. Compare readings of equipment to something known if possible - even gauges for pressures can be way off, some adjustable some not,