I agree with everyone... grab the 427, the hide-a-way's are rare and only made for the 68-69 years. You say the present engine is fine, so you can at least store the big block until you want more power...which will be sooner than you think
, grab all you can for your car.
Remember we have big car's so we need bigger brakes. I have been told the disc conversion's use smaller versions of what was factory on our's, more suitable for Chevelle's and such, but I have heard no complaints from others using them on 4200+ lbs. cars.
2nd gen Vette's are directly interchangable from spindle to dust cap EXCEPT for the steering arms which can be swapped out and although Vette's have 4 piston calipers, they are the smaller version's; not by much. Also the lower caliper mount on the Vette is differant visually, they funtion exactly. This is what I run.
They do sell the OEM units but they are pricey and really are not noticable performance wise and at half the price of Impala/Chevelle factory calipers. Plus '68-'80 Vette component's are all over and still in junk yards....oops salvage yard's as well as aftermarket.
The 255-60-15's on 15x8 Vette Rally's on all 4's with no issues.