Woa not so fast

if the source is allready ported for 64 bit programming you dont need all that.
Now it depends a bit on the compiler you use how to get a 64 bit build, in msvc with your project open click the build tab and select configuration manager.
Now set the active solution platform to x64 and hit close. You can now build a 64 bit version.
Codeblocks is a bit different especially since the default version from them has no option to set a build for x64, so with your project open in codeblocks rightclick quakespasm in the workspace view and select build options, now set the selected compiler to MinGW64 insted of MinGW and hit ok. You can now build it as 64 bit.
Small warning here, im one of the few that supplies codeblocks with both the 32 and 64 bit MinGW compilers so if you intend to use codeblocks it might be a good idea starting with that, else it would take the rest of
the screenspace on this forum teaching you how to set it up for MinGW64

.
This is about as short as i can make it but i hope it helps.
But yes if you intend to build a 64 bit executable that needs 3'rd party libraries you have to make sure those libraries are 64 bit and link to them.
As for the other stuff well int's have different sizes on 64 bit but you can get a long way with changing those to intptr_t (supplies the correct value in either mode) instead of plain int, or uintptr_t for unsigned ints.
beware that other types might also have to be changed and the compiler might not have easy types like the above for those.
Probably better to put of 64 bit porting untill you learned what the differnt types do

it will come as you play with it enough.
Productivity is a state of mind.