Okay gentlemen (and SC)... despite being away from the bench for some springtime duties, I have a small amount of progress to share on the XF-91.
I did some filing and sanding on the rear of a fuselage half to try and correct the shape of it. The area was too large. Notice the original part on the right... the fuselage lines toward the back of the plane are at an angle like they will meet each other and then they kind of straighten out and level off before they reach the exhaust opening. The left one is the corrected fuselage half. Notice how the lines in the fuselage stay at the same angles all the way to the rear of the plane. It needs a little fine tuning but it looks right. Compare to the original Thunderceptor photo below.
It took alot of filing, sanding and eye-balling to get her in that shape. 
Good thing the plastic is really thick!
Hard to see from this view but the corrected half is on the left.
With both sides finished, I hollowed out the rear. Like many planes of the era, the XF-91 had the engine tailpipe in the fuselage surrounded by the outer sheet metal.
And from what I can tell, the F-84 tailpipe should be about the right size and look to use as the J-47. If not, we'll always think that it should have been. 
I just need to figure out how I want to mount it in the fuselage and add some internal framework. Everything is rather rough right now but it will all come together eventually. 