Only 50 ---something is wrong with it, how good is the baisic engine, how well was it balanced, what is the gear ratio? ---has the dist been set up on a machine with testing of vacuum brake operation ---
The reason I ask is because I had a 39 with the 60, when it was bought it had less than 5000 miles on a rebuild and it would barely get to 50, soon after it burned a valve and got a lot of blowby ---no power, no fun to drive.
Got another block, put in nos sleeves, pistons ---crank,rods, cam ---everything in the engine NOS except block, heads, manifolds ---checked the balance, the boxed set of rods were all within less than 1 gram--big end, little end and total weight, factory set of pistons all within 1 gram ---this engine ran like new, quiet, powerful (for a 60), the car became a joy to drive---yes it didn't have the low end of the big engine---or even a stock A, but once driving was adapted to it ---no lugging, downshift as soon as it slowed below 40, don't worry about rpm, top speed was close to 80, no problem all day at 65-70 up the parkway--or across the expressway ---the only thing the car didn't like was parades, it never finished one ---20 min into the parade it would quit, after it slept for 20 min no problem going home at 70 --liked it, fuel mileage went up, before the engine 15 was best, after driving at 70 would get 25+ ---if only it had mechanical brakes, no matter how many new parts it didn't like to keep fluid in it over the winter or anytime it sat for more than 2 months.
Most of the NOS parts came from a local guy that raced boats, he said that if He kept the engine to 7000 rpm it would go 2 seasons(stock valvetrain), at 7500 it might go 1/2 season ---somewhat hard to believe, but it gave me the confidence ro run to 39 hard.
The 39 got sold ----some 10+years later I saw it in the Hershey car corral, it had been driven down from Mass, and was just sold, I walked up to hear the seller telling the new owner ---downshift early in the mountains, soon as it gets a little under 40, don't worry about driving 65, it likes it ---the first time you touch the brakes it might pull a little---I introduced myself, gave the rest of the history of the car, it had 25000 more miles, looked the same, still didn't finish parades, and hadn't had any engine work.
You should be able to get away with a 4:11 in a light A chassis, but before you consider external modifications make sure the inside is done proper ---or have you considered going back to stock A engine, properly done a stock engine can make a coupe go over 70 with stock gearing, and compression ratio.( I have gotten 23 mpg at 65 --averaged over 1000 miles)
You should start with the dist --it is very important that the vacuum brake work properly, and that the advance works --see if you can get it run up on a dist machine, check the cam timing --perhaps the gear got pressed on a little off --it can be pressed on anywhere ---before you do modifications make sure it is at least to original specs ---modifications are not a substitute for proper restoration.
A stock 60 has less torque than a stock A engine, the 60 has to make up for it with RPM
