I had a Browning 71 converted to 45/90 and I could not get 4 in the tube mag until I swapped in a shorter magazine plug. If you’re rebarrelling anyway, you might look to install a slightly longer mag tube to avoid any problem.
I agree that the shorter mag tube is more desirable for the reasons you mentioned. The other issue with heavy loads is that under recoil, the remaining shells in the mag act like a slide hammer, putting additional stress on the parts holding the magazine in place. Obviously, a full length mag tube filled with cartridges only makes matters worse.
I hope you realise that for someone who hasn’t reloaded previously going straight to reloading for a 50/110 is a tall order. Instead of a Browning 71 you might want to consider buying a Browning 1886 in 45/70 instead. This model can also be converted to 50/110 but before doing so you can use it and familiarise yourself with the rifle and with reloading. By comparison the 45/70 is easy to find components for and load data is plentiful. I’d suggest the rifle version with the octagonal barrel. When and if you decide to move forward with the conversion you can have the barrel bored and re-rifled to 50 cal, shortened etc etc. This would reduce the conversion cost and look like the original. You would need to replace the buttstock for a shotgun style buttstock - those steel crescent buttplates are no fun at all.
