If you want to use another revision of the SVN codebase,
get it and put it in a directory called dosbox-dev, then create a
dosbox-dev-0.74.rXXXX.tar.xz archive.
After that you can do REVISION=XXXX ./dosbox-dev.SlackBuild
There'll be some available at:
https://sourceforge.net/projects/slackbuildsdirectlinks/files/dosbox/

For example for the latest available revision:
# svn checkout https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk dosbox-dev
# REVISION=$(cd dosbox-dev && LANG=C svn info | grep "Revision"| cut -d' ' -f2)
# tar cfa dosbox-dev-0.74.r$REVISION.tar.xz dosbox-dev/
# export REVISION
# ./dosbox-dev.SlackBuild