I think that carburetion and fuel injection on an IR manifold are going to be very similar.
I do know a few things on throttle size from working with IDA's.
It was Shelby's Cobra team that spec'd the 48mm throttles. They raced them with 42mm auxiliary venturis, aka, chokes on the 289's. They were talking that they thought that set up became restrictive at about 6,700rpm.
The race 289 heads were called "GT40 heads". In race ported form, they flowed about 230cfm at about .600" lift.
Knowing those numbers, in late 1964 when the "light weight 427" was being released to Shelby and other factory sponsored race teams, 58mm IDA's were ordered. It's thought that less then 10 complete sets were made.
As I understand it, the testing was disappointing. None were put into production.
Testing on the 351c's for IDA's was originated for the Group 4 Panteras. The manifold was "designed" and manufactured in test form by Hollman-Moody. Ford was the customer. It was said that the molds were given to Detomaso.
The size that they determined to be optimal for that engine was 51mm. If you measure the diameter runners on that manifold, I believe that you will find they are all 51mm.
I can't tell you where restriction would be with 48's on a 351 because I'm busy holding on and the only safe way to determine that is on a dyno.
I would think that for "optimal" performance, i.e., WOT horsepower, your 427 likely would test highest with that number, 58mm? What the power curve would look like I suspect would be pretty disappointing. You would have to talk to someone like Blaine about the combination and what he could do with "tuning". That I wouldn't know. I'm a carb kind'a guy...although I did marry an "uptown girl".
Your engine is making nice numbers. If it was mine, I'd leave it alone. It certainly is pretty. I'd just worry about breaking the ZF.