Ayuh,...... Welcome Aboard,...... Donno whether yer motor or drive was swapped, coulda been either, or both,....
No doubt, you now have a mismatch,.....
The drive coulda worked with that motor, although the gear ratio would be wrong, causing over-reving, 'n lower that expected...
Ayuh,..... I donno the wire colors, but when you turn the key to "start", the slave solenoid powers the starter's solenoid, which powers the starter, 'n the outside tiny terminal of the starter's solenoid, which sends battery voltage to the points, only when starting,.....
Ayuh,....... Welcome Aboard,..... That drive will only work with another 3.0l motor, not with yer OMC motor,......
'n the SEI adapter you mention, only works on the much newer OMC Cobra drives,.....
Ayuh,..... Welcome Aboard,...... What vintage,..??
What motor,..??
What drive,..??
There's a big, like 1" hose that goes from the transom housing, to the t-stst housing,....
AD is sayin', look where it comes in through the transom,....
Ayuh,...... Welcome Aboard,....... iboats is more of a Diy help site, rather than how much to pay somebody else to fix it site,....
Besides, without knowing what's wrong, how can anybody guess,....
Ayuh,...... So do yer compression test,.......
If that's acceptable, drain, 'n isolate the block's cooling passages, 'n pressurize to 10/ 15 psi, 'n listen,......
It the block is cracked/ leaking, you'll hear the air escaping, follow the sound of the noise to the crack,....
Ayuh,..... Many, if not most grease gun couplers don't adjust tightness like in Scott's video,....
Seein's I've been greasing stuff since the '60s, I've collected alotta grease adapters, from tight right angle adapters to Cat track adapters to many others, a coffee can full of 'em,....
Now...