Weld or rivet a dip stick to the underside of the cap, ensure it has a straight line to get in the tank, the more vertical it is the easier to understand your results.

Or, one of these direct reading gauges on the tank, some mount in normal sending unit location.
http://www.rochester-gauges.be/Portals/rochester/OpenContent/Files/3436/6700.pdf
http://www.rochester-gauges.be/Portals/rochester/OpenContent/Files/3436/8680_2.pdf
another example
https://www.ebay.com/itm/NEW-Rochester-Fuel-Gauge-Sending-Unit-Float-6781-3-9-16-3-1-10-E88/360700516807?hash=item53fb6d19c7:g:RYYAAMXQigBR6b6P:rk:3:pf:0
More ideas to expand your own search
http://www.rochester-gauges.be/detail#Category:selected=EAR|paging:number=all|paging:currentPage=0



•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-