It was more of an observation, but if that’s what you needed........I’m sure a factory 12” converter could be made to live behind a motor like that.
I’m also confident an 11” could be tightened up enough to stall below 4200.
I run an 11". Works really well!
'63 Dodge 330 11.19 @ 121 mph Pump gas, n/a, through the mufflers on street tires with 3.54's. 3,600 lbs. 10.01 @ 133mph with a 250 shot of nitrous an a splash of race gas. 1.36 60 ft. 3,700 lbs.
