Jumbo's existed with both the 75mm and 76mm. Robert's correct that the Tamiya kit only comes with the 75mm gun, but 76mm guns for the kit are available from aftermarket manufaturers like Jordi Rubio.
Apparently Blast Models from France has the number one Jumbo conversion made. Somewhere someone did a review of the various Jumbo sets like the Tamiya, Tank Work Shop and new Blast set. I was able to pick up the TWS set for ony $8 from someone who had the Blast Models set inbound. I got the full set with upper hull, transmission cover, turret, and gun. I may have enough of the Tamiya Jumbo parts available for someone to do one using another Tamiya Sherman.