1. Layer of aluminum foil/Saran Wrap/Duct Tape over the pickup
2. Cone-shaped blob of clay--play-doh, plumber's putty, something like that on the aluminum foil covering the oil-pan side of the pickup
3. Spritz of lube--engine oil, spray of Pam, anything--on the bottom of the oil pan
4. Install oil pan firmly onto block. No gasket or bolts needed.
5. Remove oil pan.
6. Measure thickness of clay.
That's your pickup-to-pan clearance. Typically about 3/8 inch. Adding a proper gasket compressed when the bolts on the pan are tightened, gives a bit more clearance--perhaps 1/2".
BBC pump on a marine 454
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Having the pickup angled like you have it is NOT appropriate. Either get a different pickup tube, or a different oil pan, or discover you have excess clearance between pickup and pan.
Optical illusion on your measuring tool? Looks like it's not a 90-degree angle.