Other than the reasons you gave already. I personally don't see a reason why not...
You just have to make the decision for yourself how much you want a .223 versus how much having that 45-70 barrel around means to you. If, every time you see it, or think about it, you get a feeling of warm fuzzy attachment, then keep it. On the other hand, if you keep dreaming of a .223 and get frustrated by your lack of funds .... well you can see where this is going.
The other option of course, is do develop the discipline and willingness to delay gratification (making you a rare bird indeed
and a potential blemish on your potential as a handiholic) and you could simply wait until you have the funds.
And by the way, whatever decision you make, chances are good you may even have second thoughts about it in the future.
Just ask on this, or any shooter's forum, how many folks have had 'moments of regret' when they think of past trades or sales... as of this exact moment, I can only think of six... but with nothing to distract me and time for more reflection, I could probably spend the rest of the day wallowing in
"if only..." Cheers
Kerry