After testing your backup :
Are the modded files tpl? or source?
tpl's would be easy: simply install 1.7.4 and then copy your tpl's back
Having said that, there are some code changes that I made to my site www.damosoft.co.uk and when I upgraded to 1.7.4 there I just ran through the code putting my bits back.
However, you might want to wait a couple of weeks and check out the 1.8 release to save doing the job twice !