Halfway measures, all that.....
For the 7.3 that was in the '01 F-250 I just sold, I shoehorned in three Optima redtops. These had 850CCAs each, and served very well here where it's kinda cold...in August...and then there's January....
The Optimas are significantly smaller than standard batteries and I found that by taking out the rhs battery pan, I could slide in two of them snugger than a....I mean, snug...and after 110,000 miles that way, they hadn't budged a millimeter. Small amount of 00-ga. wiring wiring them in parallel and bingobango.
Normally, those Optimas are a tad pricey; back in late '01, I think, there was a fine sale on them, which I jumped on.
I wonder if there's enough room around the corners of the 6.4? I'll bet not - hope to find out soon!