It's an ht46 or 52. I can't tell. It's a reverse rotation unit. Problem you are going to have is this, holsets are high pressure ratio turbos. Anything under 20 pounds of boost and it's not going to be happy. At all. I have an HT60 on my N14 cummins and it makes 35-40 psi of manifold pressure daily. Honestly, your motor makes more horsepower naturally aspirated than that turbo can support.
64 merc, if you don't know what you are talking about, it would be best not to offer advice to people. The HX35 that comes stock on 5.9 cummins engines barely moves enough air to support 500 horsepower. They make a good turbo for a 2.3 liter 4 cylinder, not a 565 inch big block. HELL two of them would even be too small!!
If you want to go the junkyard route you need to be looking at Garrett units. Their wheel designs work at much lower pressure ratios than holsets. A TV8101 would be a decent starter unit on your motor to figure out what's going on. Any of the TV8xxx will work well. The TV91 units will work very well too. I had a TV91 for a while with the 7 blade compressor wheel and that unit moved enough air to support 1500 horsepower.
If you have the money to buy a dedicated aftermarket piece, you aught to be looking at something 91mm or bigger on the compressor inducer. That many cubic inches is going to need a g trim wheel and a 1.32 or 1.50 turbine housing. I'm building a big block as well except I'm at 511 ci. I have a 98 mm precision with a g trim wheel and a 1.50 housing.
Do you guys have e85 or ethanol (e100) there? Your motors static compression would make it a great candidate for alcohol. You could easily run 20 psi on that motor with that fuel and you wouldn't need two tune ups.
What does that camshaft look like spec wise? Turbo grind or no?