Swapping from a 7.25" 6 cyl rear end to an 8" rear is beyond simple... did it earlier this year and posted a thread on it. Another benefit will be the drums on the 8" rear are a bit larger- so you get a bit more stopping power.
As to "why keep the six," I've owned a 289 and a 200, and- if its a car you plan to drive a lot, the 200 is the better engine.
Good luck with the swap!
'65 coupe, 200ci, T5spd, Silver Smoke Gray, Std Red Interior '03 convertible, V6, auto, Silver Metallic, Black Top '14 coupe (on order- ETA 05/30/13), Sterling Gray, 5.0, 6spd, Track Pack, Glass Roof, Brick Red interior
This is probably a waste of time since Dan at Chockostang has weighed in on your inquiry, but you could call Currie Enterprises, Strange Engineering, or one of the several rear axle suppliers to see if they could cut you a set. Peyton Performance in Waxahachie, Texas is set up to re-spline axles. Give them a call. They can re-spline a set of 28 spline V8 axles down to the 25 spline setup you need.
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