A few years ago (about a decade ago), 205/60R15 tire options were much better. Unlike now, where it has become a touring tire size, meant for '90s Toyota Camry sedans, etc. 195/65R15 has somewhat better choices and is the same diameter as 205/60R15. IIRC, stock size for a 1985-89 740 Turbo is 195/60R15, so you might also look into the 205/55R15 size, which seems to have quite a few decent performance-oriented tires available. Speedometer will read slightly higher, due to the diameter being smaller and closer to the stock size diameter.
The other option is to go to 16" rims and run some 205/55R16 tires. Did that on both of my 240 Turbos. Won't look back.