Get FAA AC 43 13-1b, Grounding
Short answer is yes. I assume Murphy Rebel is standard aluminum semi-monoque (skin, frames, stringers). Even if its steel tube it does not matter. As far as the tail-spring anchor I guess that is a big bolt attaching to the metal frame. Sure that is a obvious sounding place. Keep in mind that with current, metal and mosture you can get corrosion. So extra attention is key.
I don't know the airframe well, but it may be better to run a ground cable forward at least a handful of feet to a different "hard point" or ground point? Not saying there is a big advantage but consider it. For example on the RV there any heavy duty stringers running the length of the plane? The two heavy duty stringers (angles) run from the tail to the firewall on each side mid way up the side of the fuselage. They runs right under cockpit sill where you get in and bolt to the firewall heavy duty fitting that the engine mount attaches to.
I highly suggest or recommend you get a copy (off line) of FAA Best practice AC - advisory circular: AC 43 13-1b, chap 11, section 15 "Grounding and Bonding". You can get a copy free off the FAA web site. (If you want email me. I'll send it (2mb file). My email address in my profile.)
You want to have similar metals attaching the cable to the airframe. You have to be careful mixing copper, steel, aluminum, brass and other metals. You should test the resistance of the ground path. They show how. For example cadmium ("Cad") or passivated bolts and washers are important. You may need to consider ground straps down stream, engine to firewall, firewall to fuselage and so on.
Of course running two big cables full length of the airframe is not weight efficient however there is new stuff, copper coated aluminum wire that has all the pluses of aluminum (light weight) with the advantage of copper (low resistance). You can purchase it at
http://www.periheliondesign.com/.
(pages on site with light weight wire
http://www.periheliondesign.com/fatwires.htm
http://www.periheliondesign.com/fatwires_files/fatwiremanual.pdf
http://www.periheliondesign.com/fatwires_files/FatwireInstallationManual.pdf
(3.08 oz/foot savings for 2 awg. 15 feet savings is about 46 oz or almost 3 lbs. Weight of aluminum cable with conductivity of copper. Down side its expensive. Do you need it? No unless you want to save 3 lbs.)
If you have heavy duty stringers or heavy frame you might can consider using that. You may need to rivet on a plate to structure to get a good attach point "LUG". If you do drill structure consider calling the factory with you mod. Drilling a several 1/8th inch rivet holes not thru the same cross section will probably be of negligible effect on the strength. You definitely don't want to drill one BIG 3/8" or 7/16" bolt hole through a stringer for the cable. Its just too much area out of one cross-section. Your tail spring bolt sounds fine but just make sure it has a good electrical path up-stream. If it going to get wet all the time, than you may want to consider another location?
Last if you have a permanent magnet starter consider getting a wire wound type. SkyTec sells the Hi-torque and the new NL model. It uses 26% less current than permanent magnet (PM/LS) starters. PM starters are great, light and cheap but they do demand a little more juice. Just a thought. You are trying to pull +250 amps. A PM starter is over 320 amps. Here is some good reading:
http://www.skytecair.com/Wiring_Experimental.pdf
Good Luck, hope we can help. Cool plane, on floats even better.
