Outdated volvo motor

I have a 78 bayliner with duel

I have a 78 bayliner with duel 170B motors and 280 outdrives. I ordered a gm straight 6 block by mistake. My question is that since the original motor and outdrive are outdated and no longer able to find is there an outdrive that would fit the new motor I have and what is it? Also would this be an extensive job putting in a new outdrive and housing to match and would the different motor ( being same size basically)mount in to the boat without to much hassle?

Thanks to anyone with answers willing to take the time to help an novice mechanic with this!
Tyson

All you have is the long block

All you have is the long block? No other parts?

Jeff

"Yes. I have all original part

"Yes. I have all original parts on boat and the extra long block and am wondering if it is worth rebuilding the one bad motor, or as I am finding out would have to replace both motros and outdrives so they match for weight and torque!

Thanks again
Tyson"