89 d-100 problems thinkin bout an engine swap

I have an 89 dodge d-100 3.9l v6 as I'm sure you all figured out by the topic line. Anyway it has a  4speed manual trans.  For some reason I am going through exhaust manifolds on the drivers side only. I had replaced it earlier this summer and seems to have gone through again. Seems to run hot in the middle cylinder where it burns a hole right through the manifold. I've never seen this before. Any ideas?

Also cause of said problem I am considering a swap to a 318 or a 360 for some speed and towing power, cause right now I can't even haul an empty car hauler more than 50mph with with the pedal to the floor and the trailer is the type with the the mesh rear gate on the trailer. My questions with this is will my rear end be strong enough for those engines? I am pretty sure my tranny won't be. And will my instrument cluster still work as well or will I have to change that all along with the computer as well? And as for my driveshaft will I have to find new or will the yoke fit right in too?
I feel stupid for asking but so far you are my only source of possible info. Thanks Bruce.