S&S crate has a S&S crank , no better choice there , and you can get the LC version which is more street friendly .. Advantage to your cases is you retain your VIN ... Either way if you sell or trade , it's a wash cause you will not get your money back .. Ask Ken (Glide69) he had a SS 124 crate and got hammered on the trade ..
I would go with the SS124 crate and be done with it ..
I totally disagree with this response sorry but I own several 124’s both off screaming eagle cases old and new T2 cases.
1st you void the warranty on a t2 case if you do internal passages, 2nd the gnurled timkens move and can’t be pinned (ruins the motor and crank), 3rd you get 2mm rings and many have oiling issues, 4th you have to carry around an MSO for the cops if they check, 5th you get the crappy X wedge gear set in the cam cover, sure you can change it but will cost you a lot more to go to old style cam chest.
If you build your own using your cases you have, there is a fault and that can be clamping force (cylinder weeping on the bottom) but using copper seal on the base baskets you can minimize this. You can get a blue pump and plate, or other, premium lifters, and 3mm rings / pistons, you can then choose the heads you want as well b2, MVA, B3, 103, CVO 110 etc.
You can also put in a pinned timken or go with 2 left bearings.
Recommend S&S crank or the P3R Crank, Drago 3 MM ring turbo pistons 0 deck 2.5 piston to wall, S&S cylinders, if MVA heads go with 2.150 valves and clean up and .035 headgaskets with 640 cam (quiet) and depending where you live ride 88-90 cc chambers with a good 2-1 pipe like Drago or Borzilla with a quiet baffle, 58 or 62 mm throttle body with 6.22 injectors, and if no lowers the heavy breather. Don’t forget a clutch upgrade this should put you around the 140’s or better.
If you decide to do a 124 LC with the 585 I would still do it off your cases the only change would be the pistons, there is about 10 of them in the group we ride with and it is a great motor combo, just be sure to keep the compression around 10.25 so you can avoid having to detune the timing and put up with pinging (PMS). Most of the bikes are in the 135-140 torque and 118-124 hp range.
If you are going to do headwork, recommend John Sachs for the job - has done all my work for years and many of the CVO guys heads and delivers on the power.
Best of luck on your build hope it turns out the way you want