Spark plug selection ??

[moparlvr4406]

My 78 SRV240 5.7 carb Merc 260
ok...the long block has been changed to a 4 bolt main marine 350 stating 280hp
on the paperwork. After a long (30-40 minute) cruise at 1500-1800 rpm it starts to miss out and lose power. also if there has been a 5-10 minute 3000-3500rpm run made. It sounds like it is loading up with fuel and fouling a plug or 6...
this only happens after the engine is thoroughly warm nearing the end of a day out. sometimes it will just die below 1000rpm with no warning.
yesterday it would not go over 1200rpm at one point and would die on accell.The temp was well within normal range.
I plan to change plugs and wires over the winter and get the Holley on top dialed in in the spring.

what manufacturer and plug number should be used on this application ??
I tend to lean toward NGK from great prior experience in auto atv and marine apps.
any advise?

 

[300SR]

I use bosch platnum's in my 454's. Always runs nice. A little more money but they have been in her for 2 years.

I also have the holley replacement for the delco 4 barrel. Makes a good combo.:smt038

 

[270SLX]

I put the irridiums in my 6.2. Must've made a difference. The idle speed doubled when it was first started up, until the computer recalibrated.