727 transmission fluid question

Hello everyone I am new to the forum and I am happy to join this active A body community. Unfortunately my first quest is not about my Duster But my 77 truck so forgive me:).Sorry if this has been asked to many times before.

So here is my question i am having my 727 for my truck rebuilt with heavy duty components and a shift kit and the transmission re-builder is recommending I use Amsoil or royal purple full synthetic at 14 bucks a quart from his shop. Are these really necessary for a quality rebuild or can i used something else I am installing the trans myself ? My dad used to use type F but it is nearly impossible to find in my area. I can get generic Dexron III/Mercon for about 4 bucks a quart generic atf +4 for around 6 a quart generic multi vehicle atf for around 4.50 Valvoline DEX-VI for 8 bucks a quart. I am in a very rural area so theirs only 2 places to buy atf :banghead: i can order fluid online but shipping is expensive. I am getting new torque converter transmission cooler and radiator as well just looking for long term reliability
The truck is a 4x4 crew cab with a 440 and weighs in at around 5500 pounds I am unsure if i need a more expensive fluid because of the the weight of the vehicle I am young and still learning about all this stuff.

Thanks