That is the same basic heater as in my 64 C10. If you do not have the flapper that was mentioned earlier then you have the Thrift Air. It has (approx) 7 1/4 x 6 3/8 x 2 1/2 core. The Deluxe has a larger core, 2" longer. My mounting plate for the core had a little ridge cast in that would mark the cutout for the larger core, I almost did the mod to convert over, but ended up not bothering with it.
I believe the DeLuxe arrangement also gives you a water flow control valve to shut down the heater, the Thirft Air doesn't have this (at least, mine didn't). Water circulates all the time, even when heater is off. The only cable controls an air flow plate that is in the portion of ducting inside the cab.
LMC has all the cores. Just measure yours to see which you've got, compare to the dimensions in the pics. I got my core from Napa (made in Mexico), for somethign like $60. Others have posted sources down in $35 5o $40 range. The cores are easy to come by, I found them in stock on the shelf at Napa and Mac's Radiator here in Portland (both had the same made in Mexico units, $5 difference in price).
Here is the LMC page with cores:
http://www.lmctruck.com/icatalog/cbe/0064.asp