To answer your question:
Yes you can run an ls crank & rod assembly in a GSR block.
You will need to tripple check the oil squirter clearance possible even do some minor filing.
You should definently use arp rod bolts for the b18a/b & have them professionaly installed as per the instructions on the arp manual.
Concerning R/S ratio:
D15B2 - 1.59:1
D16A1/A6 - 1.52:1
B16A - 1.74:1
B17A1 - 1.68:1
B18A/B - 1.54:1
B18C - 1.58:1
B20B/Z - 1.54:1
H22A - 1.58:1
You will be fine running an ls/crank & rods as long as you don't plan on running 24 hours of sebring at 8-9k
Note: Depending on what pistons you use, your machinist will have to mill 1mm out of each side of your pistons to allow the ls rods to fit properly.
Good Luck,
John