puttin in a 4.3l and a 200r4 (due to the lower first gear than a t350) any input would the stock sammi axles be strong enough for a mild built motor and 5.29 gears.

The smaller gear you go the more fragile the differential gets.  I wouldn't go any lower then the 5.12 gears, and with the V6 you probably won't even need that.

Now for the axles, it depends on tires and how you use it.  But if you wheel it or drive it hard I would look at finding some replacements.

i am goin to run 35s , i have a set of yota axles lined up so i will prob go with them.

I currently have 33 inch Baja Claw's and 5.12's with a Vortec 4.3 and 200R4, and really think going to less gear would be better for just driving and the highway. Possibly the 4.62 R&P might be about right... since my speedo and tach are both currently off, I am somewhat guessing at things, but I believe I am turning around 2800 at 70 or so, and have plenty of power to pull right up to 80+! Still need to see how she works in 4 low though, but mine is mainly a trail runner and such, not a serious crawler.

I have the 4.32004r set up with stock t/c gears & 373sin the axle & run 33-13.50 truxus tires and have had this or a buickv6 set up for 10+ years and never even thought about lower gearing in all that time Ive only broke 1 rear axle shaft and one carrier and I blame a bad lockrite install for that ... the ft has held up almost as well 2 birfields in 10 years of wheeling .. my main problems for a while was keeping the transfer case mounts hooked to the case  (but I made a custom bucket type mount )and rear ujoints