Re: Difference in Auto and Marine block on 4.3l cobra
This has been covered many times. If you search you can read for hours.
Bottom line there are minimal internal differences. If the blazer has good compression and no knocking, just use it. However you MUST use ALL of the external parts from the boat engine. Starter, distributor, carb, alternator, fuel pump etc. Also, you should replace the core plugs with brass.
The head gasket will also be different, but if you are in fresh water only you should still get a few years out of it. Of course, a complete rebuild would give you peace of mind for many many years.