It's been a long time for me, too, but I remember how smooth the old 270 sounded at 5000 rpm. The only problem was that the 11 inch Rockford clutch pressure plates of that day (mid-50s)couldn't take the rpm and kept breaking.
If you have the old Bill Fisher GMC Speed Manual, it shows a way to beef up the two center crank mains by milling flat the bearing cap and adding a one inch square steel strap across the top. The story was that with that mod, and assembly care, you could get 7000 rpm and have the engine hold together. Still seems like a whole lot of reciprocating mass.
My guess is that under 5K, the engine will last longer than maybe we will. Good luck.
Rondet
