To add to Vic’s comments, you wont find a cap much higher than 30 pounds, and then your only option is to build a system like Vic suggests. One of the things you want to keep in mind us the fin count on the radiator
In my experience the more fin count per square inch the better. I have found that 17-23 fin count, with 17 being the minimum I would use works. Without the extra fin count for support, anything lower you run the risk of core degradation. On our RMR we will be running closer to 50 pounds and on my neighbors GC Firebird, we will be running a radiator with 200 fins per inch and pressurizing to 40 pounds.
If you set up the system properly you could use different water pressure as part of your strategy to tune the motor for optimal performance. Tony