Based on some past questions on TF (and answers by folks that know what they are talking about) it appears that a good GRP 31 should be able to start your Volvo without a problem.
If your current house bank is 2 x 8Ds, isn’t that a bit over 300Ah? 4 x GC2 would be maybe 220-240Ah? Unless you don’t need much, I would think that the 4 x GC2s may give you less capacity than the 8Ds gave when the boat was new.
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