I got a 87 f250 with the 6.9 idi, she came to me with two battery’s but there pretty much toast at this point because of age and sitting so long, so does anyone know what it would take to convert over to a single battery? Maybe like a altima red top? Would this be a wise decision?

 

Can you get one Optima to equal the amperage/ capacity of two batteries

 

Will one battery start the truck??? Yes it can.
Will one battery have enough running charging time to recover what amperage you sucked out of it with your glow plugs, bowl heater (if it works), lights running, radio on, A/C blowing? Maybe if you drive a few hours at highway speed each time you start up.
Will one battery last long with the above mentioned conditions? nope.

It takes about one hour of normal driving to charge the system up from a normal start let alone a cold start where all glow plugs and accessories were on for the start.
I do short drives often but truck gets plugged in for block heater on a thermal cube, and the maintainer is always active when plugged in. If I do not do this after just doing short runs for a few days I will get a slow start.

One battery will not live long. Two batteries will not live long if you do not drive and charge them up properly. Do not skimp on the batteries, however they do not need to be fancy. Good Sealed Lead Acid from the local heavy equipment dealer will serve you well. Many folks use Wally World and are happy for a few years. 3-5 years is about normal for batteries in a diesel if you take care of them.

EDIT......
Not just the batteries... The entire charging system needs care and love. Do not overlook the alternator and cables all the way to the grounds. Yes you have more than one ground.

 

as said, one battery will not cut it......unless you live in the tropics where the temperature never goes lower than 90 degrees.
i highly doubt one battery will have enough juice to start it at 30 degrees.