I have a R/Smith also (w/o the added fuel tank, so I can't comment on that part of it, BUT adding a another battery, was hard to fit in there... BUT believe me it will fit.. take your time, grab a stool to sit down in front of it first.........and replace it.......
Here's just a suggestion: I wire/tied all the pos and neg wires first before removing them off the battery post, this will keep them all together, so your don't miss one OR get them mixed up and MISplace a wire by mistake... [the above suggestion wasn't meant to be-little you, in any way OK?] cause I myself, did it the first time I replaced a battery.....
Ronnie
3/26/22