Firstly addressing whether you should remove the square tunnel and replace with the round, for me it depends what is most important to you, the relative originality of the round tunnel or budget. I was in this position with my Riley Elf a while back, I was doing the bodywork and my budget was limited. At the time my priority was getting the car solid and running within my budget so was going for the cheaper square tunnel floor. At this point I lost my workshop and had to hand my car over to a restorer for the bodywork. With my budget already blown because of this change I decided that the body should be as original as possible and agreed for them to remove the later parts and re-do with a round tunnel.
The round tunnels are available from M-machine, see section 320 of the catalogue in the link. Catalogue price is shown as £120, but this will likely have changed due to steel price fluctuation recently.
http://preview.m-mac...i catalogue.pdf
On the subject of fitting the later rod change box to this round tunnel I have just done this as my engine/box seem to be from a mid 80s car. As has been mentioned above you'll need a conversion bracket
http://www.minispare...|Back to search
To get the rod change to fit in the tunnel hole without modification I have then fitted an adjustable top engine steady so allow me to adjust the engine tilt
http://www.minispare...|Back to search
And a longer rod & steady to the gearshift mechanism as fitted to the post 85 1275 cars.
http://www.minispare...c/UKN10032.aspx
https://www.minispor...lector-rod.html
All this allows the shifter to sit in the hole, slightly forward of the centre point without cutting the shell. I'm afraid I don't have any pictures of this, will get some this weekend, but it all seems to be fine and gears seem to engage so far, but the car isn't running yet.
Edited by humph, 05 May 2021 - 08:31 AM.