Hi Snowman.
Owned both VM and 3B powered BJ73s. I've owned two VMs and currently drive the 3B.
They drive very differently. The VM is a great deal more 'sporty'... if thats the word to use. It has shorter range diffs so its much faster off the mark. If you can get the VM running well its a cracking engine, and REALLY easy to work on. It suffers from legendary turbo lag but if its tweeked and you drive accordingly its got plenty of power.
The 3B with longer range diffs feels slow by comparison. Slow off the mark and its definitely slow up hills. The 3B will take a turbo conversion though. I haven't gone down that road as even with only 4 gears at 55mph it returns 30mpg.
If the VM engine is shot then by all means look at a replacing it, but if it's running well I'd leave it in.
If you need new cylinder heads theres a guy in the UK (i can dig out his details) that sells brand new OEM heads for £90 each. Use 1 piece steel CH gaskets if you can... you'll need end plates shaved down to accommodate .
The engine mounts are different, as are the gearbox input shafts and Rad.
My 3B has a gearbox fault (no fifth) and I have a spare H55F gearbox on the bench waiting for a rebuild. It came off the last VM and needs a different input shaft. The 3B version has (I might be wrong on this) 21 splines and is about an inch and a half longer. The VM has ten splines and is quite stubby. Antway, they're definitely not the same.
There is a brilliant thread on IH8mud buy a bloke who swapped a VM for a 3B.
The BJ73 is an awesome truck. Good purchase.
Iwan on here has a great thread...
https://www.landcruiserclub.net/community/threads/bj74-1hdt-h55f-combo.151177/
If you're going to do an engine swap this is the way to go.
keep us posted
Dan