Your best bet would be to just sell your gun and buy a Commander. There are differences in the Government and Commander frames, so if you went with the shorter slide, modification would have to be made to your Government that would be permanent. A good slide will cost you $200.00 and a decent barrel is at least $100. If you want a properly fitted barrel, then you're looking at $175-$200 for the barrel plus the cost for a gunsmith to fit it. Then there's the cost for the gunsmith to modify your frame. So a conversion from Government to Commander would be in the neighborhood of $500.00 if you want it done right.
Just put your government model up for sale, and you may have to kick in a C note or two for your new Commander. The money saved would pay for a few very fun trips to the range.
.400 Cor Bon looks nice on paper, until you have one. The ammo is VERY expensive and it's not the most fun round to reload. If you like the ballistics, then sell your government and get a 10mm instead of a .45.
Spot on having done exactly what you desire 20 years ago.
