Most likely the same bolt pattern, but the 4630 is a 3 cylinder and the 6600 is a 4 cylinder.
Look at the parts book for differences in the mountings. They made mounts for both tractors for the 7410.
https://www.messicks.com/nh/141796
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That is a casting number for the block...raised letters.
Early NAA models the serial number was on the left front of the engine block just below the head
Later it was on the flat area of the bell housing just behind the starter.
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I am assuming it is a 3 cylinder 4000, 4100. Here is a parts breakdown of the cylinder.
Go to section 06, then 03D02
https://www.messicks.com/nh/138355
Is there a gearbox brand that you can see? Did you try:
https://www.dnequipment.com/store/pc/viewbrands.asp
OR
https://www.brooksagparts.com/product-page/replacement-hardee-finish-mower-gearbox-code-15340-1-2-83-ratio