what timing cover and indicator are you using? there are three different locations for tdc on balancers and three different locations for the tdc indicator. you need to find tdc for #1 based on the piston and see where the balancer and indicator are.

To confirm the timing mark you have to bring the #1 piston to TDC and verify the mark on the balancer is lined up with the mark on the timing pointer. If your harmonic balancer was for a different engine design the mark could be in one of 3 places. The good news is you can just paint a mark on there.
If you think the crank is not spec, look up the part information by it's part number in Scat's info, or call their tech line. You never know.

You are right that the carburetor is too big for the 350, Unless it has vacuum secondaries. If it has them then the carb will self adjust to the intake air volume.
Go back and reset the idle jets to 1.25 turns from the seats. Time it to eight degrees BTDC at 750 rpm and then adjust your idle jets (probably down) for best running at 750 rpm with proper mix.
Now bring the RPMs up til the advance maxes out. Don't let it pass 32 degrees. I think that distributor gets it's all in advance at 2800 or 3200 rpm. If you do this on the trailer don't keep it revved too long, Just be up there long enough to get a number. If the distributor doesn't get 32 degrees you can adjust it for all-in timing.
Then, set idle to 750. Final adjust the mixture by turning in one idle screw til you get a stumble then back out a bit til it's smooth. Adjust the idle back to 750 or 725,Now set the other idle mix screw. Reset the idle rpm as needed.


I bet you have so much fuel running through there that it takes more timing to stay running. I've done it to myself, so I thought I'd throw this idea out there.

what did the harmonic balancer on the engine come from?some of the trucks used different timing chain covers and balancers due to placement of accessories.my guess is you have mismatched cover and balancer.

you said the balancer and cover are factory correct?? I think that's what you said...the only variable has to be the crank then...my guess is its keyed slightly different than the factory crank...that said..just get the motor to TDC and paint mark the balancer to the pointer showing true 0...doesn't really matter that the marks don't line up..just make a new one...that's my suggestion for what its worth lol

The outside of the harmonic balancer possibly slipped around the inside. Not sure I'd use a 29 year old harmonic balancer

A Holley 4160 is a vacuum secondary carb. Same as 4150 except use the secondary plate controlled by vacuum.
The spring for the vacuum control can be changed to adjust the secondaries.

I did not read any of the this -- just the headline, sorry if it has been discussed. just adding maybe some helpful info. Make sure cam timing is correct / TDC and you have the correct sized balancer. Make sure it did not slip either. ( the outer vs inner of the balancer ) If you are using timing TAPE make sure the tape is for the correct sized balancer. Man do i see that one alot being wrong. ie: . The timing tape is sizes specific per balancer. If you are using a pointer make sure it is correct for the correct balancer,

Sorry I've been away, been working alot, gonna work on it tomorrow roll it around to top dead and recheck the marks, if it's a crank key way issue ,I'll just remark it and go from there, if the balancer itself did spin ,I'll replace it..it still looks fairly new, but ya never know, I did have a couple good back fires with the old merc distributor, maybe it slipped, and if every thing lines up...then ..... idk.. I'd really like to get it timed right. i know this motor could run way better and push my boat faster than 50 mph and get it out of the water alot quicker , it does run way better after down jetting the cab ,just this timing issue, thank you for all the great info, I wil lk definitely try out the tuning timing processes mentioned and get back with everyone with what I find

I did have a sbc balancer spin on me. Rubber got old and it moved. Maybe compare key way to another balancer in the shop to see if it is close?

Off the subject and not anything to do with your timing marks, but what are you running for exhaust? stock manifolds and risers? That cam is really pushing the limits on duration for reversion and then you added a few more degrees effective duration with the 1.6 rockers. Your hesitation and stumble may very well be some reversion. Just trying to save you more headaches.