Nion,
I was in the same predicament. I originally bought my Z as a non-runner, planning to get it running and that's it. Well, the 'while I'm at it' bug bit me and I bought another to rape for parts.(I paid less than $400 each). Well I ended up getting the second one going with little effort and decided to drop a V8 in the first. Well as you can see by the pictures, I went 'a little' further than just dropping in the motor and trans! . But, it is a LOT easier to do what you want while the car is completely stripped, than to go back later and do it.
Anyways, as to your questions.....
The straps have a 'toothed' loop thingy that 'locks' the belts in place unless you pull the other way. I have tried them and the hold well. I also have a 'lid' for the cell box, which will be on hinges (on the side closer to the dash) and a latch on the taillight side. That way once the carpet is in, it will be 'invisible'. (except the outlets sticking out under at the back).I also plan to put some foam around the outside of the cell (between the cell and the box)just to be sure that with a full tank it doesn't slide around under hard cornering, stops and pull offs.
Let me know if you need any more info.
Tim