I am replacing the oil cooler pipe work on my 7 in preparation for a new engine fit.
I have been told that 5/8 pipes are the ones to go for.
I have talked to Rally Design, they say the Mocal thermostat sandwich plate (SP1) has 1/2" BSP threaded out puts.
I can get 1/2" to 5/8" adaptors .
but I am concerned that I will have a restriction at the adaptors.
Do you know of a Mocal thermostatic with 5/8" ports or will the adaptors and 5/8" pipes be ok.

When I built the V8 for my Cobra, I had a thermostat sandwich plate from Think Automotive. The restriction was the orifice which is opened by the heat, not the fittings. I modded it to make the orifice as large as possible. Then rang them to point out the flaw in the basic design. They agreed, and said since I ordered mine, they had gone with a different design that they would send me FOC. The replacement was better, but still the restriction was the orifice rather than the fittings.
It would be worth checking that within the whole system, that the 'bottleneck' is the 1/2 inch fitting. I would be surprised if it is.
I do not know what engine you are talking about, but even with my 6.3 Chevy some argue the oil rarely needs cooling except on track days.

I am running a 2.1 pinto, no bhp yet but seriously modified and running 8,000 rpm.
Heat always a problem in a 7 and the oil cooler on the last engine helped.
So the 1/2" to 5/8" adaptors not a problem in your opinion?