To add my .02, last fall when I began my ls swap into my 69 Nova, I bought a set of hooker plates that utilized a standard small block motor mount. My car had a late model 5.7 sbc and had the "clam shell" mounts. I took those off and used sbc towers and mounts. Put the motor in, everything fit great....until I went to buy headers. Found out the mounts I had wouldn't work with what I had- same header company that made the plates-I had to buy a new set of mounts that use the clam shell mounts.
so long story short, had a done the research, I could've bought it all at once and not had to pull the motor back out to put new mounts on. Lesson learned I guess