73 spool trans mount/ Drive shaft Blues..

Okay, measured drive shaft, it is 47.375"- 47.25" CL of U joints. (UGH)
Looks like its time for a new driveshaft. I did some calling and I was asked about my engine size, I know this was being used to know the wall thickness of tube to use.
So now I have another random question, how much HP is safe to put to a stock 318 drive shaft? I know my 318 won't hurt a stock one, but kind of curious incase someday I want a little more power and drop a 360 in that might have 500hp max (Maybe)
Thoughts?