Engine woes

["Myron Ware" <cmw@domain.elided>]

Hello all,
  Here's my saga:  In a fit of insanity, I tried to draft a big rig 
traveling faster than my indicated 70MPH (152 4-cyl, 4.27 gears on 
31" tires), and my motor decided to frown upon this act after around 
20 miles of slipstreaming.  There was no smoke, just a loss of 
power ans it shutdown as I was coasting on the shoulder.  There are 
no oil leaks on the ground, and no milky oil on the dipstick.  The 
oil pressure is fine when I start it up, and it sounded like it 
always had before getting towed home.  I had taken it on short runs 
through the neighborhood to test it and all seemed well, so I took it 
to get a Christmas tree and about four miles into the trip, it 
started happening again.  I limped home and last heard what sounded 
like a spun bearing or two right before dying again.  I started it 
yesterday but I can hear that something isn't quite right with it.  
The weather hasn't been conducive to a teardown, so I haven't pulled 
the pan to check the bottom end.      
  I'm thinking of replacing the motor since it had thrown a rod (and 
was repaired) before I bought it.  I've got a line on a 196 with a 
4-spd and Dana 300 that's been recently rebuilt and is waiting to be 
installed in something.  I currently run the 152, 3-spd 
(non-hydraulic clutch), and Dana 20.  Is this just a 
drop-it-in-and-attach-the-driveshafts swap?  My steed is a '68 800. 
It's my understanding that the 196 was just a bored and longer 
stroked 152 block, thus the motor mounts should line up as they are.  
I'm looking for reliability at this point and have even considered 
*gasp* transplanting the heart from a late-model GM donor.  I know, 
"The horror!", but the little 152 barely gets out of its own way, so 
I'm looking for something with a tad more pep.  I'm in Va, and the 
rebuilt drivetrain is in SW TX, so I want to be sure of all options 
before I commit to the final choice.  Any and all suggestions are 
welcomed. Thanks and sorry for the longwindedness, 
Myron '68 800 4x4, 152 4-cyl (RIP?)