Bought a truck, replaced battery… but sometimes truck wont start…?

Bought a truck... I parked it up for a couple of days. It wont start.

The truck is a 90 ford ranger 2.9 liter, but the previous owner converted the engine to a 98 4.0 liter engine.

Im not sure if two things are going on... but I believe the flywheel is for a 2.9l starter... so I have problems starting the truck and other days it fires up beautifully. Sometimes the truck stalls in public places and wont start up again.

If I park the car for more than 5 days, the battery (which I replaced... brand new) will die. Often times I will have to recharge it.

What do you think is going on... Im thinking two things... because the stereo (which the previous owner replaced) suddenly died, and I notice a battery drain

and secondly the starter... seems to have problems turning the car over.

I have taken it to two shops... both have charged me for diagnostics, but none have found a problem... despite the car not starting.

Frustrated... what to do???