iv'e got a 28 sonic with 350 mags 300hp.the top rpm is 4900 how long can i run at 4900 and can i crusie at 4100 all day?what is a good crusing rpm?

Keep an eye on your temp gauge if running that speed with a load.

temp 148-150 oil preasure 75

how long can i run wot with out damage under a normal load and how long at 4200

Real hard to answer your question. Too many variables to pin it down exactly.

1. Do you have an oil temp gauge and what temp does it run?
2. The harder (more rpm) you run the more wear and tear on parts period.
3. Service - oil changes, If you run hard then more frequent changes should be done.

I have a 502 mag that has been run at 3800 to 4200 cruise. It has over 450hrs with no problems. I would think a 350 mag would do the same with no problem.

WOT for long periods well if your treat it like a race motor expect to rebuild pretty frequently.

I am not sure the 350 mags have oil coolers and I can not stress enough keeping the oil temperatures down to resonable level along with frequent oil changes to help keep them motors running. Just my .02