So I looked at putting the heater aft of the rad and fore of the alternator, just no room there.
Plan C, theres lots of room ahead of the main radiator, but the downside is the hose from the heater to the engine is about 17inches, longer than we like, too much heat loss between heater and engine. After I get it together, I will wrap the hot side of the hose with fiberglass+foil and see how it goes.
This is the heater I'm putting in, I see its 1000 watts not the 750 I thought. Not a bad thing, since the install situation is not ideal and there will be more heat loss than usual due to the extra-length of hose from heater to engine.

I took the cover off the heater to make sure nothing was burnt, corroded or leaking or otherwise degraded from its previous install. This is whats in there. I mention thermodisc, for me thats a generic description of a bimetal disconnect device:
49T High Temperature Bimetal Control | Emerson

This is the large metric fine thread plug, the 3/8 18tpi npt tap, and the 3/8 npt barb fitting

This is the mount of the heater in front of the rad and oil cooler, from the top

This is the mount of the heater in front of the rad and oil cooler, from the front - sorry about the sun flare

This is the hose from the heater, past the left edge of the rad, and over the alternator and into that fitting above the thermostat

The pictures show no clamps on the heater, I was just trial-fitting at that point, I still had to take it all apart and put some shrouding back in place, at which time I put the clamps on.
I pressure tested it all, and it held 15psi for 15 minutes with no perceptible drop on the needle

I put the coolant back in, had to add a pint because I spilled and piddled while draining.
Plugged it in and waited. A few minutes, and the hose going into the top of the engine was hot. By the time I got all the tin back on and all the stuff put away it was an hour, the back of the head was warm to the touch and it started without glowing.
Albeit it was 5C outside, I think thats about 40F. Forecast says -15C in a couple days, we shall see.