What do you use to cap the holes in the rear of the longs where the heat exchanger hoses are normally attached?

Spray paint caps fit perfectly. I used them to keep media out while blasting the car.

Yup. Secure with a hose clamp and it almost looks stock.

2” black plastic pipe caps fit perfectly - use some glue to secure them.

I deleted my heat as well and went a different route. I wanted something a little more permanent but had to be removeable if needed so I found these SS caps that the beer brewers use on their equipment. I drilled a hole in the center and used a SS Allen head bolt. I took a 2.5" piece of steel about a half inch wide and welded a nut to it. The steel fits across the hole, inside, and the bolt snugs it nice and tight. I brushed on seam sealer as a "gasket" and then brushed on seam sealer like the factory and partially covered the cap. Looks pretty clean and it will be painted the same color as the car. I can't find any installed pics but will look again. Here is what I bought, and almost a perfect fit. The groove on the back fits on the lip of the holes.

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Cut off bottom of a pop can and a thin bead of RTV, I use them for big and small holes just cut to size. Only meant to be temporary, but a preheater hole on my T1 engine has one that must be 25 years old now.
I also use paint can lids where they fit.

@BPic and you call me "Mr. OCD"?

yup

I don't think my holes are capped.... what's the down side?

You don't want critters and funk in your hole(s)

Pop cans.

https://www.lowes.com/pd/Fernco-2-in-dia-Fl...ings/1000075369

@CoronaMike
Dead rodents and nests in the longs. Ask Jamie Rust.

+100, thanks for this recommendation

You mean a soda can?

Yes...and it's not a beanie it's called a toque

Hosers! Take off, eh?

Sorry off topic, have any of you converted your seats to electric...Mark