Rear end question

[JJchamp48]

This was my setup :
Factory was IL6 with Powerglide tranny.

Set up when I got it:
350/th350, 4.11 rear end. (I'm assuming tranny & engine were changed at same time). Engine had been rebuilt approx 14 months earlier.

We drove 100 highway miles, blew engine.
Don't know speed because dash lights didn't work. Ditto on rpms. Based on speed of my other car I was following, figured I was going about 45.

Fuel sending unit leaked, so had to ride w/ windows down to get rid of fumes, plus straight pipes out back were extra loud. Rpms sounded higher than they should have been.

There was a spare tranny in truck when we bought it because, get this,...the truck was running rough and cut off, so the guy replaced the transmission. This didn't fix the problem, duh, which was rust from the tank sucked through carburetor.

My truck has since been in that catch 22 where, no need to get engine built until rear end is done and it's down to cab & chassis right now so need to sand & prime/paint. And winter was here, no fun working outside in cold & snow.

We just recently pulled tranny from truck. It is a TH350. Just checked #s on spare tranny and it is a 700R4 (8THM097H), w/ 'K' on housing. From what I can tell, it's 88 from 4x4, tailpiece was changed to 8", but some ID pages say if the ID# has an 'M' third letter, it's a 4L60. Either way it's an OD tranny. Had just assumed that trannys were the same.

Set up we were going with:
68 Vette 327, 3.42 rear, TH350 tranny.

This truck will not be used for hauling or towing, I have a 3/4 ton GMC for that. It will be strictly pleasure. Will be making trips to visit family, races, show, etc. Won't be a daily driver. The only thing we may tow is a trailer we want to make out of a SWB frame. Driving highways/interstate. Speed range will be 55 - 70 mph.

First plan was to change rear gears. So got a 3.08 rear for smaller carrier and 3.42 gears to put in it.

From what I've read, instead of changing rears, I can use an OD tranny and the 4.11 rear will be good for highway/interstate driving.

Sorry for the long post, but wanted to get as much info in post. I'm sure someone will ask me something I haven't included, but hope most of the info needed is here.

So now I will turn it over for your thoughts/suggestions.

Thank you in advance!