I bought the Defi D-series gauges, and bought a Defi Sandwich adapter (fits just like you said, between block and oil filter). I bought mine from Godspeed Motorsports
. Dan is a great guy and his business is great. He has the Defi and Greddy unit.
And I found that the gauge sending unit did not screw into the sandwich adapter. Great. Well, it came with an adapter, and that didn't work either. Double great.
So, I went to McMaster-Carr.com and ordered 2 each of a bunch of adapaters that might work. I had heard that Defi units sometimes are BSPT, or BSPP, or something different from what I had.
And I came up big time. Two of the ordered adapters (1/8" FNPT ----> 1/8" MBSPT adapter in 316 SS) worked perfectly. The sending units screwed perfectly into the adapters, and the adapters fit perfectly into the sandwich unit. Added some teflon tape, torqued 'em in, and I was good to go on my next oil change.
Only thing I had to do for the temp sender was drill out the adapter a bit, as it did not have a large enough ID to fit the sender. Nothing the Dewalt drill and bit couldn't help.
The McMaster-Carr part # is the following; 4092 K11
They're $5.68 each, kinda pricey, but I haven't been able to find BSPT around me at all.
Hope this helps.