55 3200 drivetrain questions

[Msanantonio]

I have a 55 second series3200 (longbed) with a 350 chevy crate engine Edelbrock upgrades, a saginaw 4spd and the original HO33 rearend. Based on the serial # the truck originally came with the small v8 and the 4spd and what I beleive to be 4.10 gears. We spent the better part of the spring getting it back on the road and have been having a blast driving it this summer. Need some advice on the tranny and rear gears. I drive it a lot and sometimes travel at highways speeds engine is at 3300 rpm at around 65 mph. There are some drivetrain issues that I want to address in the fall. there are a lot of pros and cons to the saginaw (hp limitations ,durability gearing vs. availability and low cost to fix) does anyone have any thoughts on a good transmission /gear combo?

At the very least I am planning to open up both and refresh them as necessary as the tranny has a noise in 4th gear a rattle not loud and not at a high frequency. There is a also vibration (shudder) around 22-2300 rpm that goes away it's not gear related. This has the original 2 shafts with the center bearing.
Pinion angles look ok and clutch is not slipping.

Would like to keep the original rearend thinking the cheapest fix is to put a set of lower gears. I think the options back then were 3.38,3.90 and 4.10. I am not in favor of the 3.38 but it may be the easiest way to drop the rpm sat highway
speed.

The other options? I'm thinking is a 5spd . Not looking to break the bank at least for now. Thanks for your feedback.