Don't ignore the fact that the iron head has no rockerbox cover, the valvesprings are pretty much exposed, which means that cooling air can get MUCH deeper through the head.
But you've got a couple of alloy Mac donks there so you'll probably want to work with them. (It may be the better downdraft angle with the iron head that is good).
I was advised by a couple of ol' blokes (when I wasn't an ol' bloke!) One was Eric Bushell If I remember rightly. He is (was) a Mac racing guru. The other feller was a Tasmanian who had very good results in Tassie.
I'm still searching for Eric's letter, but my above post explains the important stuff. Yes a Triumph piston -can't remember which, fits and ups the compression ratio, I think I got around 9.?:1.
My carb was 1 1/16" same as Viper - and I'd reckon viper settings would be a good starting point.
I think I used a Viper exhaust valve (Nimonic 80) - It might have been turned down).
I timed at 38 deg with no Manual
adjustment - fixed timing using a Criterion alloy mag pinion. W & S Valvesprings. Roller (converted) clutch release bearing. I think I fitted a 7 plate clutch (KSS) but not sure about that.
That exhaust... (well you asked for opinions!) - Rip off the kickstart assy (they're pretty useless anyway due to low gearing and big overlap of the M17/8 camshaft). Put on a banking plate and bump-start it (or make a roller paddock starter) -Then you can use a KTT bend or maybe a Manx Norton
bend, both of which should tuck in much better and give good clearance. (And look dead sexy into the bargain!)
I can't quite see what gearbox you're using, if it's iron Mac it will have the selector lever on the front face. Early MSS / KSS will have the selector on the rear face.
The MAC gears are narrower, but if they're in good condition I don't see why you couldn't use them... the 'box would be a bit lighter eh? - and bear in mind my earlier point about adding a second top gearbox mount if it's the iron MAC shell.
You'll need (at least one) good brake. The 7" SLS Velo brake can be very effective, but I'd certainly machine relined shoes to ensure complete drum contact.
If I think of anything else I let you know. Reminder - I am no expert, and I'm relying on unreliable memory!!!
I think it's good to see a younger guy doing a Velo project rather than 'bobbing' CX Hondas!