The DVLA ( Driver & Vehicle Licensing Authority I think ) holds the records for all vehicles and drivers in the UK. I guess the equivalent of your DMV ?
When you import a vehicle you have to get it registered with them to obtain a UK registration. To do this you have to insure the vehicle and get an MOT ( roadworthy ) test, although pre 1960 cars no longer need an MOT test.
As far as I know i should get an age appropriate number which should be 3 letters and three numbers. This is the same format as the number which the car had in california which was 352 TUY, and that fitted the plate fine. There is apparently a website called statesplates or something similar where you can get US style plates made.2
Although there are regulations as to the size / shape of licence plates and the text fonts etc. there are exemptions where a standard UK plate wouldnt fit etc. Also the police are apparently quite lenient with classic vehicles. I would hope they'd never seen a squarebird before and would be far to interested in the car to worry about the plates
It is torture having the car and not being able to take it out on the road hehe.