ahhh. ok. well if its a big block .21 it will take a lot of modifying to get it to fit a stock chassis. I've seen it done by moving the tranny up one hole on the chassis and doing a lot of modifying. you are looking at around an extra inch of length on a big block vs a smallblock. Most .21 conversiosn chassis are streched by at least 1 1/4" to 1 3/4".

the stock clutch on a maxx will fit right on a big block with the right shimming. the diamaters of the crank snouts are the same just the .21's are longer overall than the 2.5's. Easy fix is just to grind a mm or two off which is common on all big block .21 maxxes.

just for size reference the engine on the far Left is a my old traxxas Pro .15 (same motor mounts and case dimensions as the 2.5) all the others are .21's or larger.

Big difference there ehh?

there are .21's that have the same footprint as the 2.5 and will bolt right in
like this one. that would be your best bet if you want a .21 in your maxx without buying a conversion chassis. I'd just upgrade to a sirio .18. if you go .21 on a maxx it breaks everything. trannys, diffs, the entire driveline is too weak for the torque a .21 puts out. thats the other major problem with .21 maxxes.