Good Choice

He he he, I bet u drool so much reading all the Riffe great stories Sven & I put up.
As you said it, cable must be the way to go in the sharp edged rigs. You may want to try Riffe coated s.s steel cable. They have 500 and 270 lbs, I suggest u try 500 lbs cause ur 3/8 shaft is thick enough to carry the drag. 2 standard wraps should do but if you try an empty shot at water, beware...ur shaft's momentum will rob ur gun. Coated is better because it is very kind on the and and do not scratch the anodized finish of the MT,also a bit stronger. Use electrical shrink wrap where the cable( & crimps ) is exposed. Double crimp is how I do on my Riffes cause I used Riffe supplied crimper ( cheap & great ), not the pro table based swager, so I use double always as insurance.
Maybe you want to buy a 3M dull black sticker or any colour you want. This can be pasted all over the aluminum anodized finish and should further protect ur gun from scratthes. These stickers are quite thick and strong. Won't add anything to the weight. If urs come with the plastic muzzle, you can always get the aluminum silver muzzle if you want to use 4 x 5/8 rubbers.
If later you decide to shoot Wahoo and want super shaft speed, you may buy a 7mm Riffe Hawaiian ( also try 8mm later ) shaft and shorten it like Sven to get better accuracy.............even with 4 x 5/8 rubbers. Shooting line and length ( 2 or 3 wraps ) wise, ask Sven.
You will be surprised all these three shaft sizes may make your gun feel, swing and customized different, for suitable games.
Remember not to aim JBL style. Do some practice before u aim at anything. Whatever your JBL model is, a 3 x 5/8 rubbers on an MT4 already need two handhold........with practice you will live with a single hand, if u ever require so. At 4 x 5/8 she need left hand on butt properly, if u r right hander. What you have now is not just a speargun, that is a mini cannon, won't loose power wise to a Wong, unless they come up with a Kim or a Nguyen
Now for the moment we been waiting for...................our very Own Mr Smooth....SVEN.......let's hear it from you :
In the mean time good luck Rig.
IYA