I would go with a bigger engine, you will be disappointed in my opinion going from a 427 to a 327. I would also not do the Orion. I would do a split case with 4:1 gears. It's quieter.
I haven't researched gm transmissions for a while. From what I remember the 4l60e variant of transmissions can be built quite strong. As in like 700ftlbs of torque. So personally I will probably just go with a performance built 4l60e variant if I kill my transmission. Currently in my rebuilt...
A gm transmission is so easy to replace I consider any worry of a 4l60e not lasting forever as a almost non issue. You can always upgrade to a 4l80e easily if needed or buy a beefed up 4l60e.