Can you put a standard later M28 spider and exhausts on an M4 engine?

Afraid not, absolutely incompatible.
Sam Fieldhouse holds rather a lot of shares in the M4 spares stockpile, if it's a servicable M4 manifold that's needed...

Many thanks - I was just thinking I've got a lot of M28 bits and the thought of the exhaust bits of an M4 and their routing fills me with dread - could you put m28 heads on an m4 block then?

You can put M4 heads on an M28 block thats been done.

I know I'm an M4ophile, but to be absolutely honest, instead of fannying around, just invest in getting it set up proper like and they're dreamy engines.

but soooo slooooow!

Just a thought - I've got a set of nearly new 9:1 Pistons - would they fit as a direct replacement to the originals?

No, the bore diameter is different. Even if you changed the barrels, id be hesitant to subject the crank to that - 7.75:1 - 9:1 is a big leap.
Love it for what it is, it's a different driving experiance to the later cars!

True - tbh I don't get much chance to drive it as Scoobasteve always has it at his - I'm just the grunt who fixes it! It's just a massive difference jumping for that into my 652 2CV that I keep pondering about giving it a boost.....

I can't remember what carb you said it had? The 40 does give extra oomph.
My van will do 60 quite happily and, if I really go for it, it's hit 69 - good, but in comparison to you 652 I imagine still sloth like

Umm,
cylinder bore is the same as later 602s at 74mm, it's the shape of the piston crown which is very different.
There doesn't seem to be much information given on the Snail forum about how an M4 top end was grafted onto a later bottom end; I've got all the necessary bits here, just lack of time.
It wouldn't be quite the same, that's for sure.
Even if the 'mix and match' engine's CR was kept around 8.0:1, I'd bet that an M4's camshaft profile has a lot to do with its 'relaxed' behaviour...