Just had a look at the Traxxas site and the piston/sleeve is different between the 2.5 and the 3.3 so I doubt that the 3.3 head will fit the 2.5 bvut give Bob Davis a call to confirm this anyway.

5230X Piston/sleeve (matched set), wrist pin clips(2) (TRX 2.5, 2.5R)
5290 Piston/Sleeve (Matched Set), Wrist Pin Clips(2) (TRX 3.3)
(Use with crankcase #5225)

But as I said, there are other diesel conversion head suppliers out there.