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I've got a 55 GMC second series half ton w/ the original 248/hydramatic. The block is shot on the 248 and I'm looking for an easy drop-in/swap using the stock tranny/rearend. What adjustments/mods have to be made to drop a 270 or 302 in where a 248 was? Thanks!

Faster speedometer..

Very few issues for they are essentially the same motors with a larger bore and stroke. You may find that you need a different generator mount. I know that a 228 and a 270 do not use the same mount.

Should be an easy swap..

What about the hydromatic - seems like I have always heard that some "match" machining is done to properly mate the engine to the tranny. Is this the case here?

The flywheel rear housing is supposed to be bolted to the tranny housing and checked for square using a dial indicator..

I spoke with Patrick Dykes about this issue once, he told me that GM made a bigger deal out of this that was warranted..

They used to drag race these trannys in the 50's. I kind of doubt they were all matched to their original engines..

I've never seen a 302/270 with hydro

248/228s cranks have less bolt holes in the flange
I think your need the 270/302 flex plate
If it's all like the stick stuff you have to change the ring gear to use the 228/248 starter

I had one of those tranys on the back of a 508 Lincon

The late 248s had a 6-bolt crank flange pattern so you "should" be OK there.
The 302 had a couple of different front motor mount setups. There was a big "horse collar" mount that bolted to frame somewhere, and some allegedly used side motor mounts like a V8 though I've never seen an example. 302s came in a lot of big GMCs like busses and commercial chassis, and many others in military vehicles. They never came in pickups, thus the front motor plate behind the timing cover is just a flat piece of steel, it won't have the angled central motor mount like the 248 has. The 248 plate is a bolt-on, but beware of interference with the front damper pulley, if you use the stock 302 double groove pulley. You can turn down the rear face of the pulley for clearance, or change the whole balancer out for something more compact. Even though the engines are the same dimensions, the 302 took up more room due to the accessories. But in general it is pretty straightforward swap.

Thanks to all for the great info! I'll keep you apprised of my progress, and recount any difficulties I may discover.