That only works with the early 1 piece t-cases, not the split cases.
For an SM420 -> Split case you need a different adapter. Advance Adapters USED to offer a SM420 -> split case adapter, but you lose a 3-4" of driveshaft, though the adapter...
I did an SM420 15 years ago or so.
A SM420 is super cool in a 40.
With an index ring and a few other small parts, it bolts up to the 3 speed bellhousing.
Gears are 7:1, 3.5:1 1.7:1 and 1:1 Basically doubles your crawl ratio.
You used to be...
If it's already V8 converted, I'd get an LS and run that upgrade so you can get fuel injection and a modern engine. I'd go with the 4 speed or a 5 speed at the same time if you can fit that into your budget.