My best friend in high school had a 61 Impala 348 solid lifter 3 twos (350 hp) 4 speed with 4.11s. It ran strong enough to stay side by side with a 63 425 hp 409. That car was incredible for what it was. The 350 hp engines were very rare I once saw production numbers which I don't remember but there weren't many built.
The other option is finding and obd1 ecu and making an adapter to hook it up to the obd2 engine. then modding the obd1 ecu to be flashable.
Edit: Just saw it is a obd1 already, then you should be able to mod it, no need for aftermarket ecu.
I have a question to ask about my 620 I put everything on the outside of my engine I don't know what to do it will miss one minute and then start running right the engine has a whining noise just wandering if you might have any ideas what my problem might be.