I grew up runnung a 1967 squareback 55. It was a gas and gear drive. With a 2 row 235 CH you could shell 300 bushels an hour without unloading on the go. We had extensions on the grain tank to hold 100 bushels.
I bought a nice 55EB last fall for $750. Pricey but not everyone has your grandpa's combine. I did 50 acres of soys, 10 acres of corn with it last year and 22 acres of oats this year. I remember when I was a boy that this combine was huge. When I went to look at it I couldn't believe how tiny it was.
The roundback 105 had the 248 running fast, later the 341 came along with the 362 gas last. The diesel engines were the 404, they sure seemed to outpull any 362 I ever had. Good ole combines for sure.