Thanks cleverusername. I know Dolly's had 'J' types and the prop will be shorter.
Like most on this forum, over the decades I have 'played' with cars. My 13/60 Herald estate has a Mk IV Spitfire engine in (I picked up a brand new short block, cheap from a dealer clearout) It also has a 4 synchro Spitfire box with 'D' overdrive, I shortened and balanced the prop myself but I have a spare brand new non O/D Vitesse prop which is the correct length. Ratios aren't really an issue if you can accurately fix on the under or over recording of the speedo. Plus twin 125 Strombergs, wolf in sheep's clothing
I've also just finished an engine swop on my daughter's 1960 Standard 10 Companion, the mains and big ends were starting to knock, for a Herald 13/60 engine and box (she wanted an O/D fitting but I said no). Block and box lengths are the same. Along with making a new loom (which I've done a drawing for).
I was tempted to put a Mk IV 4 synchro box in my Dolly, I have a spare but the rear half of the mainshaft is for an O/D box so I would have had to put an O/D in.
All this has made me realise I MUST WRITE THINGS DOWN, then they can go with, and be part of the history of the cars so that others don't have problems in the future!
Thanks again everyone, I've learned a lot over the past couple of days, especially with RobSun's thread.