Why? The six is just fine--if you put a faster 8 in there, you'll need to really check all the handling items and probably improve them--stiffer springs, sway bars, etc. Might need to lower the suspension, etc. By the time you are all done, you don't have a 113 anymore--so just get a nice car with a V8. I'm sure the Good Doctor and Joe Alexander might have some comments about adding a V8 to the 113 Chassis.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored