[Brian]

Yoyodyne, To try and answer your question, I think [my opinion], the limiting factor is breathing. I have tried an L100 in a 221...I'm not particularly impressed with it. Sorry Tony!
Most everyone on here that is flathead performance [oxymoron] orientated, believes [possibly rightly so], that there is no substitute for cubic inches [apart from maybe rectangular money], but I tend to prefer developing the little 221 on account of its propensity to rev, on account of its smaller, lighter rotating/reciprocating assembly.
I've spent a major period of my life trying different engine mods, fitting said engines into the same car each time, on that eternal quest of finding out what works best.
Gearing plays a big part in the equation too...generally, I've found using the standard 28 tooth cluster and a 3.78 ratio out back, is a good combination for all round street performance.
So, how to build a sweet 221? main objective is to let it breathe! This involves the usual valve work, porting, multi carburation etc. Next is to raise the compression. Herein lies the old challenge of getting compression up without sacrificing breathing. Aim for 60cc volume in the chambers or less. Now, if you fit your desired L100 cam you'll find the valves in all probability hit the sparkplugs... the way to get maximum breathing through those convoluted passages is to raise the roof, BUT also narrow it...the chamber profile does not follow the head gasket contour. Do not relieve the block [obviously]
OK, how else do we improve the revvability of our favourite little engine? By lightening the flywheel/clutch assembly. Than get the whole kaboodle balanced.
I cannot tell you which cam to use...as previously said, I don't believe an L100 to be best choice....a stock 32 steel cam is pretty good [they have hotter timing, subsequently detuned by Ford in 35 with the introduction of the first 'fat Fords, which need more torque]. I find an Isky 88 to be a good upgrade. That's what I use. There are so many cam choices, it really becomes up to the builder of the engine, taking into account the application of the engine, and the mods done to it, to make a selection.