A quick personal note. I haven't got the part numbers or specs you need, but do have a couple of lines of inquirey.
1.The stock replacement Cleveland 351 2v valve springs for the first 1970-1971 265 hp Torino/Mustangs will work okay.
2. The old Crower single spring, once listed as #68315, will work to 6500 rpm, if it was possiable. It has lots of nose tension, and is as big as you'd ever need. You don't need twin or triple valve springs. At 5400 rpm, you won't get any spring surge.
The rule with all cams is normally to use the minimum spring tension you can get away with out valve float and lifter pump up. The X-flow has a 20% lighter valve train than a 351C, but with with heavier, longer pushrods than the Cleveland. It is heaps heavier than the non cross flow I6's. The valve springs used on the 351C Fords are therfore okay for most mild to wild X-flows.
A very important note on long term durability. Since all canted valve Fords suffer from extreme valve loads because of the heavy valve gear, I'd look for valve springs which will still hold considerable tension in 10 years time. Engineer Lee Morse from Ford said that the canted valve Boss/Cleveland/Lima/Modified suffer valve death if they are not oiled well. On a high lift cam, I'd say dial up heavier valve springs as they will be worked harder than on similar sized Chevys or 2V head engines. There is about 8% more lever arm, and up to 5% more valve weight to lift than any regular log 144-250, or 2V or Argie I6 engine. X-flow and Clevleand valves are longer and bigger in most dimensions than even the last big log 200/250 engines.
Use the Isky #347 -ST Chev big block retainer for the multiple grove valves Ford uses. Multi grove valves are not the strongest. The hardened Ford Cleveland/Big Block 429/460 10 degree keepers (collets over here) can be shaved back so the spining mechanisim doesn't work. Stock keepers on Fords are designed to butt up together, and allow rotation of the valve to eliminate hot spots. Under really high revs (above 6500 rpm), you may want to go single grove. Below that, I'd modify the mutli grooves to stop the valve rattling. If you focus on things which break, and fixing them, then you can go easy on the valve springs. Extra heavy valve springs will cost performance and create cam and chain problems. Like all Detriot Fords, Aussie ohv Falcons suffer from a small link pin cam chain, and extra tension on the cam is to be avoided. Ford added more links to cope with the heavier valve gear in 1971. The cross flow got and upgrade too.
Don't go for springs too heavy!
I know it can be done...right? I've been told that the real trick is to not lose the valve down into the cylinder! 
Some tricks/suggestions for that?
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