Your wheels are called 'Omega' I think (part no. 1343663) and were originally fitted to the 940.
The centre caps (part no. 1358923) were also used on 3 other wheel styles of the same age.
Be aware that the centre caps that you have are missing the circular wire spring (part no. 1325483) that sits in the groove on the inside rim of the tabs on the back. The spring holds the tabs against the wheel and stops the centre cap falling off.
As for tyre size, your 740 would have originally been fitted with 186/65/15 tyres so 195/60/15 would be the closest match for circumference, to allow your odometer to read accurately. On my last 940 I had 'Draco' alloys that were the same size as yours and the tyres were 195/60/15.
I found a simple tyre size calculator
HERE that can help you decide.