The floorspace on a -800 737 is very close to any -135. If you really want the length, a -900 is longer volume much better. Electronics and all are smaller and more efficient these days, the P-8 is beyond good enough to replace a 1950's airframe.
First, a lot of the NRE for the P-8 is already done, that's huge. I've worked certification, military and civil, the "paperwork exercise" consumes vast amounts of money. Second, the -800 platform gets to a higher altitude faster which gives a much better radar horizon which can matter for certain wavelengths. Third, two 1990's engines burn far less gas than four 1980's engines, operating costs matter these days. Finally, a 737 fuselage is based off of the 707 fuselage which is bigger than the -80, KC-135 fuselage. The extra width gives more room for equipment, operator stations, cooling, and the large fuselage compared to the biz jets used these days for the function allow for bigger, more sensitive receivers.
As I noted above, the 707/727/737 148" diameter fuselage is all of 4" larger in diameter than the 144" KC-135 fuselage (actually, both are larger in diameter than the 367-80's 132" diameter fuselage).
Indeed, no intent to ignore your contribution, merely a summary of facts already in evidence combined with a lot of flight hours in -135's and as a passenger in -800, -900, -8 Max, and -9 Max over the last two decades.
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