Does Tamiya model cement or Ambroid Pro Weld actually melt and fuse the plastic of HL tanks or do I need a PVC type glue?

HL is not a PVC. CA glues have worked well enough for me in the past. Anything that really needs strength you would want to use an epoxy any way. Hope this helps.

I have used Plastruct Bondene successfully with HL plastics before - the label doesn't say what's in it, but it smells like some other methyl ethyl ketone (MEK) based solvents I have used in the past. Pro Weld makes similar claims about its uses, so you should be ok with that.

HTH,
Mart

I've done a great many conversions with HL tanks and CA glue does fine. For stubborn or hard to line up parts, you can use a drop and hit with Zip Kicker accelerator to lock it in place then add a couple drops to adjacent areas for strength.