Shannon:
I have asked this same question - I have an L with a '67 440 in it. With my car, I am choosing not to stress out about it too much. A) - I don't have unlimited resources, and B) - an L is definitely the least prized of the Letter cars. Besides that, I'm not too sure how many "HP" blocks you can find out there.
Having said that, an F is much more valuable in general, even more so with the original motor but, with a "date correct" block (as long as it's disclosed) I can't imagine you detract from a "numbers matching" value too much. Just an observation and opinion. If you have the time, and patience, you may be able to uncover the right block.
If you are on a schedule that doesn't allow a year or two to look, perhaps you take a stab at it briefly, and then just move forward. Who knows, maybe a "V41" is the right date code for my early build '65 L and you have me as a customer for yours? Perhaps it would be in my best interest to help you look?
Good luck in whatever you choose, and keep my name in case you do want to sell the "V41" block.
Dan Richardson
300L Family Heirloom