[Mike Taylor 3601]

As far as I know could use 1pc or 2 pc seal block and be legal.
1 pc. uses different flex/flywheel is external balance on rear and has different bolt pattern.
I have converted several to internal balance takes about 1'' heavy metal usaully depending on bobweight.
the reason they are external balance on the rear is when flywheel flange is made round it loses that extra flange that is counterweight
Mike Taylor 3601