The info for the Grace Majestic says that it will support machines with up to an 18" throat. Why can't you just use your A1 head on the grace frame? I would call the grace company and just double check.
My 18" tin lizzie is on a grace imperial 10 foot frame. The rails are 5 feet long pieces and are joined in the middle. From reading the info on the Majestic, it looks like that is also the case for that frame. The basic rails are 5 feet long and a 2 foot extension is available. You can have 2 five foot units, which would make the frame 10' or add a 2' extension to the single 5' unit to get a 7 or to two 5 foot sections to make a 12 footer.
I'm pretty sure that all the frames made by Grace have 5 foot rails that you join. Don't think they make a one piece 10 foot unit.
I would not say that it's mechanically challenging to change the size, but it sure it a pain in the butt, as you have to remove the machine and totally disassemble the frame.
One more thing to consider is that you loose about 5 or 6 inches on each side of the frame to accommodate the sled, so a 5' frame would only have a useable area of 48-50". To do a twin, you might want to consider the 7' set up.