Stick with the PPG, for now and once it gives way then invest in the sixspeed. This will give you time to locate a good sixspeed in your time frame.
The Powerglide is a great box, most of the big power pickups in Texas use them(up to 1200 bhp) but the added cost of additional works will made a sixspeed the more cost effective way forward. Plus the sixspeed is proven where as the the Powerglide will only be as good as the modification work.
Hard one to call but upward and forward
Powerglide at work