have you actually tried the pr4? you say it's an obd1 pr4...92 ls came w/ pr4. 93-95 came w/ p75, but both should do the job. 96 came w/ a p75, but it's obd2a and will not work for your application.

if you need a p75 in particular i can socket/chip the pr4 and load p75 maps into it.