Depending which rod and how far down the slug you put your ring pak (EX: a nitrous slug tend to lower the rings a bit), you may not get into the pin area. Example A 4.25" stroke low deck with a 6.535" rod comes up to around 1.30-1.32" compression height, depending on MFG and final deck height, In fact it can typically take the same piston as a 451/6.76" rod low deck combo... (that's the beauty of the 511). But a 4.15/2.375" journal crank with a 6.76" rod will definately need a support rail, again it's no big deal. I generally prefer the 4.25"-4.30" combo all else being equal for the added cubes and particularly the smaller BBC size 2.200 rod journals which aid in keeping the bobweight down...not to mention less clearence grinding in the block....even with the longer stroke.
Guys sometime cite the 6.535" rod as being too short of a rod ratio but consider a 4.25"/6.535" combo 400B based 511 still has a (slightly) better rod ratio than a typical 4" Stroke /6.135" rod 454/468" BB Chevy of which there have been countless THOUSANDS of them built and run in bracket cars at very high RPM's for FORTY plus years!