Greets!

515s have somewhat different specs than the later B, C, etc, instead being closer to the 515-8Gs [higher Fs, lower Vas], so not a good match-up if re-coned unless GPA has a parts combination to get close enough specs and HF BW ?tone? as there's no more originals available.

There's also the question as to whether GPA can still re-cone/re-zap to 'powerful' motor [low Qts] specs since lately there's been some folks getting relatively 'weak' motored [medium high Qts] re-cones. This isn't necessarily a bad thing if one plans to use a large cab or OB and drive it with a typical SS amp, but tubes, especially SET, need a powerful motor to keep from being under-damped in operation.

Yes, the 511 can be cut down to where the throat area is the same as the driver?s exit = ~1.54?^2 and add a mounting flange same as the factory did. Easiest/most accurate I guess would be to get AB to make an accurate length measurement from the front of one of his factory conversions.

Anyway, all things considered, unless you just have a keen desire to keep the 288Bs and good 515s, then consider selling them since they historically bring big $$$, even blown ones in some cases to fund new, or at least later, versions depending on the needs of your app. Depending on the parts condition and market, you might even wind up with enough extra to completely fund the project.

WRT horns, consider using two 511B/1? driver/speaker as I once did to replace 805/288B to ?have my cake and eat it too? since I wanted the fuller mids of the 805 and the extended HF of the then current 802. This combo actually makes for a bit larger effective horn and driver, though never having compared the two side by side, can?t say if it?s audibly noticeable, at least with a 500 Hz XO.

GM