I am getting ready to possibly do a conversion on a 190D. What size is the high size fittings on these? I have bought fittings before and the high side adapter was too small to screw on. I can't remember what size I bought in order to buy the other size.

Should be the same as the low side , 1/4" flare.

Usually, US designed cars used the smaller ( 3/16" ) high side connection to prevent improper connection.

Thanks!