I can think of numerous people that would probably love to be in your prediciment! Seriously, while every species has its pros and cons, I personally believe this is a very good choice for a frame. The fact that the timber is old growth and has tight graining actually helps reduce any shrinkage or movement. The reason that such timber is sought after by flooring and furniture makers are the same reasons that make it very suitable for frames.....in fact, flooring and furniture are more critical applications where shrinkage and wood movement need to be controlled (boy, I bet I get some responses to that comment!). Most, if not all, flooring and furniture crafters require a good stable material with <6% moisture content (requires kiln drying)where framers can get by with something less (higher moisture content). I believe your timbers would classify as SYP for structural purposes and probably exceed any strength requirements. Again, consulting of a structural engineer in this area is usually a good investment. Any negatives to using your timbers would be additional weight but certainly not anything that cannot be managed and increased difficulty of making joints (probably similar to a red oak) - again, these are not problems but just differences over using "newer growth". Sounds like a good thing you have going and I would expect it to make a great and beautiful frame. Only consideration would be with the prices that many people are getting for old growth pine is to sell it for that purpose but obviously that is a decision for you plus I hate to think of timbers getting sliced up. Best of luck!
Caveat: these are only my opinions and I am certainly not opposed to anyone having better knowledge telling me I am incorrect (in a nice way of course).
Roger
