273-4, 10.5:1, and a stock spec cam. What's the strongest single spring that I can install and make my peak red line of 6000 rpms and be repeatable?

Comp 901-16 is go to with that size cam --it is a single but can be set up to enough pressure easy
The 995-16 is the go to SB double for big hyd's and again pressures can be adjusted but hey if you have 130 ish and 300 ish you will be plenty good for that RPM fact is 125 and 275 range will actually do it but up to 325 is OK with extra care on break in etc
I would just do a 901-16 set up at 125-130 and then whatever open is with your lift that will be safe for lobe wear and still run great

I like the 942-16 comp springs for stock-ish cams in SB mopars. With a couple valve jobs installed height is around 1.700. The 901 needs a .06 shim. With a .06 shim the spring wanders. The 942 works with stock retainers and will have around 115# seat and 280# at .480" lift, without shims.

I have to ask.....is this an OEM "stock spec" cam or an NHRA "stock spec" cam?