You should be good with a size 29 deep cycle or even a 31, they are VERY close in size, most manufactures don't even make the size 29 anymore and just jump up to 31 (like Interstate)
Deka/Duracell recommendation chart based on outboard size: http://www.dekabatteries.com/assets/...ionslayout.pdf
Deka/Duracell battery specs chart: http://www.eastpennmanufacturing.com...Sheet-0194.pdf
Based on that chart for a 60 hp, you need a minimum of 380 CCA (502 MCA)
Interstate 24 cranking battery that you had, assuming it's a 24M-XHD
CCA - 800
MCA - 1000
RC min at 25 amp draw - 135 min
In going to a deep cycle, you lose some of the cranking amps in exchange for more reserve capacity, which is what you seem to be needing more of.
If you stick with Interstate, just going up to a SRM-27 you get
CCA - 600
MCA - 750
RC min at 25 amp draw - 160 min
Or if you had the room (the 29 and 31 are so close in size, wouldn't hurt to double-check the dimensions), SRM-31
CCA - 675
MCA - 845
RC min at 25 amp draw - 210 min
I think the best value right now is the Duracell Group 31 Model# 31DC at Sam's Club for $100. It's the exact same battery as the Deka DC31DT
CCA - 650
MCA - 800
RC min at 25 amp draw - 185 min