Can this be done by keeping all the accessories and just swapping out the long block with a new 454/502 long block with carb?

If you are re-using flywheels and balancers you need to make sure you have internally or externally balanced and don't interchange them. I would also have some concerns about reusing the oil cooler from the 330hp. May not support the additional btus. If an intake is not part of the new long block, you will need rectangle port to match the mag heads. If you are reusing the carb it will need to be retuned. There are probably other areas I am missing, but that should get you started.

Dave

Besides what Dave said, Some of the accessory holes need to be drilled out from 3/8" to 7/16".

The 7.4 are 1993 vintage so fairly new . Sounds like it might just be better to get complete new motors and sell the old ones.

I swapped a Gen V 7.4L for a Gen V 502 Mag. I found a new (used, $50) flywheel. The motor came with a balancer. I went with a used (another $50) Performer RPM rectangle port intake. The oil cooler upgrade is definitely necessary. I would recommend at least a 12" X 3" (cooling area, not overall length) cooler. The stock carb will work but, a Holley 800cfm works much better. It is a pretty straight forward swap. because of the different intake manifold you might want to relocate some things, such as the Mercathode and the main breaker for a cleaner look. I am very happy with the swap. I picked up an honest 8 mph in my 260 Baja.

I have two merc 502 rect port intakes with the brass inserts in great condition if you need to purchase.

Dave

MACDAD260

What did you use for a motor? Did you go with a GM 502 marine block? Did you have any difficulties mounting the accessories ( sea pump, power steering, alternator on the new block?
Dave

BLUETHUNDER-

If I buy the boat I probably won't do the conversion until the end of this upcoming season.
Dave

easy switch....

the bellhousing,engine mounts , exhaust are drop in for the 502 no problems however; all the accessories from a MK IV 7.4 will not bolt onto a Gen 6 454/502 mag

1. you will need the larger oil cooler and seperate P/S cooler and all new hoses since they take a different route rather than the small combined oil p/s cooler on the 7.4. The small oil cooler from the 7.4 is not large enough, don't try it unless you boat at Frobisher bay .

2. The raw water pump is different ,the Gen 6 is shorter and so the bracket will need to be shimmed about 3/16" or you will need to get a gen 6 pump.

3. the alternator brackets etc use 7/16 vs 3/8 bolts in dif locations and will need to be re-drilled

4. the Gen 6 block does not have provision for the 3rd foward mounting screw on the the starter motor altho one can be fabricated from a brass plug.

5. the flywheel and harmonic balancer are different if the crate engine does not come with them.

ursus-
the 7.4 is a gen v motor.