I reccomend 9/16" and bracing your spring pad. If you have any power and / or hook, these factory parts will fail. I obtained my 9/16" u-bolts from an RV spring supplier.
NHRA #7494 SS/H
1970 AMC AMX
"Best Appearing" @ NHRA '06 Pacific Sports Nationals
JeffLeeAzRealty@Qwest.net
602-418-9797