If you have that much HP in a heavy car with sticky tires I would get a hemi box. But Make sure your getting the 18 spine bushed gear sets from passion.
I build 4 speeds and Dan from Brewers never steered me wrong any info. I would call him and explain my dilemma and he would always have the correct answer to solve the issues I have. If he was out Cindy would always have him call back, He took time called me and talked to me on the phone.
I have several Hemi box here including A-body. They sit on the shelf. Heads up and Gassers race cars are usually 4 spd users. Big HP bracket cars go automatic.
I have put Hemi engines and 440's in 383 street cars with 23 splines . The weak link was always the 741 pinion shaft on the 8 3/4 rear . A good 742 is what worked. Or a 489 with the early 741-742 clutch style power lock. The 69 and later cone style sure grips are throwaways and usually slip if one tire sticks and the other don't.
View attachment 1716300180
View attachment 1716300181
View attachment 1716300182